…d builds

It affected cmake-unity shared-curltool curldebug mingw-w64 gcc
builds when building the `testdeps` target.

Apply the solution already used in `lib/base64.c` and `lib/dynbuf.c`
to fix it.

```
In file included from curl/lib/version_win32.c:35,
                 from curl/_bld/src/CMakeFiles/curl.dir/Unity/unity_0_c.c:145:
curl/lib/memdebug.h:52:14: error: redundant redeclaration of 'curl_dbg_logfile' [-Werror=redundant-decls]
   52 | extern FILE *curl_dbg_logfile;
      |              ^~~~~~~~~~~~~~~~
In file included from curl/src/slist_wc.c:32,
                 from curl/_bld/src/CMakeFiles/curl.dir/Unity/unity_0_c.c:4:
curl/lib/memdebug.h:52:14: note: previous declaration of 'curl_dbg_logfile' with type 'FILE *' {aka 'struct _iobuf *'}
   52 | extern FILE *curl_dbg_logfile;
      |              ^~~~~~~~~~~~~~~~
curl/lib/memdebug.h:55:44: error: redundant redeclaration of 'curl_dbg_malloc' [-Werror=redundant-decls]
   55 | CURL_EXTERN ALLOC_FUNC ALLOC_SIZE(1) void *curl_dbg_malloc(size_t size,
      |                                            ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:55:44: note: previous declaration of 'curl_dbg_malloc' with type 'void *(size_t,  int,  const char *)' {aka 'void *(long long unsigned int,  int,  const char *)'}
   55 | CURL_EXTERN ALLOC_FUNC ALLOC_SIZE(1) void *curl_dbg_malloc(size_t size,
      |                                            ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:58:48: error: redundant redeclaration of 'curl_dbg_calloc' [-Werror=redundant-decls]
   58 | CURL_EXTERN ALLOC_FUNC ALLOC_SIZE2(1, 2) void *curl_dbg_calloc(size_t elements,
      |                                                ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:58:48: note: previous declaration of 'curl_dbg_calloc' with type 'void *(size_t,  size_t,  int,  const char *)' {aka 'void *(long long unsigned int,  long long unsigned int,  int,  const char *)'}
   58 | CURL_EXTERN ALLOC_FUNC ALLOC_SIZE2(1, 2) void *curl_dbg_calloc(size_t elements,
      |                                                ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:60:33: error: redundant redeclaration of 'curl_dbg_realloc' [-Werror=redundant-decls]
   60 | CURL_EXTERN ALLOC_SIZE(2) void *curl_dbg_realloc(void *ptr,
      |                                 ^~~~~~~~~~~~~~~~
curl/lib/memdebug.h:60:33: note: previous declaration of 'curl_dbg_realloc' with type 'void *(void *, size_t,  int,  const char *)' {aka 'void *(void *, long long unsigned int,  int,  const char *)'}
   60 | CURL_EXTERN ALLOC_SIZE(2) void *curl_dbg_realloc(void *ptr,
      |                                 ^~~~~~~~~~~~~~~~
curl/lib/memdebug.h:64:18: error: redundant redeclaration of 'curl_dbg_free' [-Werror=redundant-decls]
   64 | CURL_EXTERN void curl_dbg_free(void *ptr, int line, const char *source);
      |                  ^~~~~~~~~~~~~
curl/lib/memdebug.h:64:18: note: previous declaration of 'curl_dbg_free' with type 'void(void *, int,  const char *)'
   64 | CURL_EXTERN void curl_dbg_free(void *ptr, int line, const char *source);
      |                  ^~~~~~~~~~~~~
curl/lib/memdebug.h:65:30: error: redundant redeclaration of 'curl_dbg_strdup' [-Werror=redundant-decls]
   65 | CURL_EXTERN ALLOC_FUNC char *curl_dbg_strdup(const char *str, int line,
      |                              ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:65:30: note: previous declaration of 'curl_dbg_strdup' with type 'char *(const char *, int,  const char *)'
   65 | CURL_EXTERN ALLOC_FUNC char *curl_dbg_strdup(const char *str, int line,
      |                              ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:68:33: error: redundant redeclaration of 'curl_dbg_wcsdup' [-Werror=redundant-decls]
   68 | CURL_EXTERN ALLOC_FUNC wchar_t *curl_dbg_wcsdup(const wchar_t *str,
      |                                 ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:68:33: note: previous declaration of 'curl_dbg_wcsdup' with type 'wchar_t *(const wchar_t *, int,  const char *)' {aka 'short unsigned int *(const short unsigned int *, int,  const char *)'}
   68 | CURL_EXTERN ALLOC_FUNC wchar_t *curl_dbg_wcsdup(const wchar_t *str,
      |                                 ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:73:18: error: redundant redeclaration of 'curl_dbg_memdebug' [-Werror=redundant-decls]
   73 | CURL_EXTERN void curl_dbg_memdebug(const char *logname);
      |                  ^~~~~~~~~~~~~~~~~
curl/lib/memdebug.h:73:18: note: previous declaration of 'curl_dbg_memdebug' with type 'void(const char *)'
   73 | CURL_EXTERN void curl_dbg_memdebug(const char *logname);
      |                  ^~~~~~~~~~~~~~~~~
curl/lib/memdebug.h:74:18: error: redundant redeclaration of 'curl_dbg_memlimit' [-Werror=redundant-decls]
   74 | CURL_EXTERN void curl_dbg_memlimit(long limit);
      |                  ^~~~~~~~~~~~~~~~~
curl/lib/memdebug.h:74:18: note: previous declaration of 'curl_dbg_memlimit' with type 'void(long int)'
   74 | CURL_EXTERN void curl_dbg_memlimit(long limit);
      |                  ^~~~~~~~~~~~~~~~~
curl/lib/memdebug.h:75:18: error: redundant redeclaration of 'curl_dbg_log' [-Werror=redundant-decls]
   75 | CURL_EXTERN void curl_dbg_log(const char *format, ...) CURL_PRINTF(1, 2);
      |                  ^~~~~~~~~~~~
curl/lib/memdebug.h:75:18: note: previous declaration of 'curl_dbg_log' with type 'void(const char *, ...)'
   75 | CURL_EXTERN void curl_dbg_log(const char *format, ...) CURL_PRINTF(1, 2);
      |                  ^~~~~~~~~~~~
curl/lib/memdebug.h:78:27: error: redundant redeclaration of 'curl_dbg_socket' [-Werror=redundant-decls]
   78 | CURL_EXTERN curl_socket_t curl_dbg_socket(int domain, int type, int protocol,
      |                           ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:78:27: note: previous declaration of 'curl_dbg_socket' with type 'curl_socket_t(int,  int,  int,  int,  const char *)' {aka 'long long unsigned int(int,  int,  int,  int,  const char *)'}
   78 | CURL_EXTERN curl_socket_t curl_dbg_socket(int domain, int type, int protocol,
      |                           ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:80:18: error: redundant redeclaration of 'curl_dbg_mark_sclose' [-Werror=redundant-decls]
   80 | CURL_EXTERN void curl_dbg_mark_sclose(curl_socket_t sockfd,
      |                  ^~~~~~~~~~~~~~~~~~~~
curl/lib/memdebug.h:80:18: note: previous declaration of 'curl_dbg_mark_sclose' with type 'void(curl_socket_t,  int,  const char *)' {aka 'void(long long unsigned int,  int,  const char *)'}
   80 | CURL_EXTERN void curl_dbg_mark_sclose(curl_socket_t sockfd,
      |                  ^~~~~~~~~~~~~~~~~~~~
curl/lib/memdebug.h:82:17: error: redundant redeclaration of 'curl_dbg_sclose' [-Werror=redundant-decls]
   82 | CURL_EXTERN int curl_dbg_sclose(curl_socket_t sockfd,
      |                 ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:82:17: note: previous declaration of 'curl_dbg_sclose' with type 'int(curl_socket_t,  int,  const char *)' {aka 'int(long long unsigned int,  int,  const char *)'}
   82 | CURL_EXTERN int curl_dbg_sclose(curl_socket_t sockfd,
      |                 ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:84:27: error: redundant redeclaration of 'curl_dbg_accept' [-Werror=redundant-decls]
   84 | CURL_EXTERN curl_socket_t curl_dbg_accept(curl_socket_t s, void *a, void *alen,
      |                           ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:84:27: note: previous declaration of 'curl_dbg_accept' with type 'curl_socket_t(curl_socket_t,  void *, void *, int,  const char *)' {aka 'long long unsigned int(long long unsigned int,  void *, void *, int,  const char *)'}
   84 | CURL_EXTERN curl_socket_t curl_dbg_accept(curl_socket_t s, void *a, void *alen,
      |                           ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:93:28: error: redundant redeclaration of 'curl_dbg_send' [-Werror=redundant-decls]
   93 | CURL_EXTERN SEND_TYPE_RETV curl_dbg_send(SEND_TYPE_ARG1 sockfd,
      |                            ^~~~~~~~~~~~~
curl/lib/memdebug.h:93:28: note: previous declaration of 'curl_dbg_send' with type 'int(SOCKET,  const char *, int,  int,  int,  const char *)' {aka 'int(long long unsigned int,  const char *, int,  int,  int,  const char *)'}
   93 | CURL_EXTERN SEND_TYPE_RETV curl_dbg_send(SEND_TYPE_ARG1 sockfd,
      |                            ^~~~~~~~~~~~~
curl/lib/memdebug.h:98:28: error: redundant redeclaration of 'curl_dbg_recv' [-Werror=redundant-decls]
   98 | CURL_EXTERN RECV_TYPE_RETV curl_dbg_recv(RECV_TYPE_ARG1 sockfd,
      |                            ^~~~~~~~~~~~~
curl/lib/memdebug.h:98:28: note: previous declaration of 'curl_dbg_recv' with type 'int(SOCKET,  char *, int,  int,  int,  const char *)' {aka 'int(long long unsigned int,  char *, int,  int,  int,  const char *)'}
   98 | CURL_EXTERN RECV_TYPE_RETV curl_dbg_recv(RECV_TYPE_ARG1 sockfd,
      |                            ^~~~~~~~~~~~~
curl/lib/memdebug.h:105:30: error: redundant redeclaration of 'curl_dbg_fopen' [-Werror=redundant-decls]
  105 | CURL_EXTERN ALLOC_FUNC FILE *curl_dbg_fopen(const char *file, const char *mode,
      |                              ^~~~~~~~~~~~~~
curl/lib/memdebug.h:105:30: note: previous declaration of 'curl_dbg_fopen' with type 'FILE *(const char *, const char *, int,  const char *)' {aka 'struct _iobuf *(const char *, const char *, int,  const char *)'}
  105 | CURL_EXTERN ALLOC_FUNC FILE *curl_dbg_fopen(const char *file, const char *mode,
      |                              ^~~~~~~~~~~~~~
curl/lib/memdebug.h:107:30: error: redundant redeclaration of 'curl_dbg_fdopen' [-Werror=redundant-decls]
  107 | CURL_EXTERN ALLOC_FUNC FILE *curl_dbg_fdopen(int filedes, const char *mode,
      |                              ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:107:30: note: previous declaration of 'curl_dbg_fdopen' with type 'FILE *(int,  const char *, int,  const char *)' {aka 'struct _iobuf *(int,  const char *, int,  const char *)'}
  107 | CURL_EXTERN ALLOC_FUNC FILE *curl_dbg_fdopen(int filedes, const char *mode,
      |                              ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:110:17: error: redundant redeclaration of 'curl_dbg_fclose' [-Werror=redundant-decls]
  110 | CURL_EXTERN int curl_dbg_fclose(FILE *file, int line, const char *source);
      |                 ^~~~~~~~~~~~~~~
curl/lib/memdebug.h:110:17: note: previous declaration of 'curl_dbg_fclose' with type 'int(FILE *, int,  const char *)' {aka 'int(struct _iobuf *, int,  const char *)'}
  110 | CURL_EXTERN int curl_dbg_fclose(FILE *file, int line, const char *source);
      |                 ^~~~~~~~~~~~~~~
```
Ref: https://ci.appveyor.com/project/curlorg/curl/builds/49840554/job/a4aoet17e9qnqx1a#L362

Ref: curl#13689 (CI testing this PR with `DEBUGBUILD`/`CURLDEBUG` combinations)
Closes #xxxxx