All mingw-w64 versions declare these IDN function, but some mingw-envs
set the default _WIN32_WINNT value conservatively, e.g. to 0x0502. In
such case idn.c was assuming an old SDK env, assumed these declarations
missing and declared them. This caused duplicate declaration warnings.
Fix it by always skipping local declarations for mingw builds. (Classic
mingw may fail on that, but curl no longer supports classic mingw)

I'm not sure why this wasn't happening with cmake builds in the same
build env.

Seen on Ubuntu mingw-w64 cross-builds.

```
../../lib/idn.c:154:23: error: redundant redeclaration of ‘IdnToAscii’ [-Werror=redundant-decls]
  154 | WINBASEAPI int WINAPI IdnToAscii(DWORD dwFlags,
      |                       ^~~~~~~~~~
In file included from /usr/share/mingw-w64/include/windows.h:73,
                 from /usr/share/mingw-w64/include/winsock2.h:23,
                 from ../../lib/setup-win32.h:91,
                 from ../../lib/curl_setup.h:308,
                 from ../../lib/idn.c:29:
/usr/share/mingw-w64/include/winnls.h:1075:30: note: previous declaration of ‘IdnToAscii’ was here
 1075 |   WINNORMALIZEAPI int WINAPI IdnToAscii (DWORD dwFlags, LPCWSTR lpUnicodeCharStr, int cchUnicodeChar, LPWSTR lpASCIICharStr, int cchASCIIChar);
      |                              ^~~~~~~~~~
../../lib/idn.c:159:23: error: redundant redeclaration of ‘IdnToUnicode’ [-Werror=redundant-decls]
  159 | WINBASEAPI int WINAPI IdnToUnicode(DWORD dwFlags,
      |                       ^~~~~~~~~~~~
In file included from /usr/share/mingw-w64/include/windows.h:73,
                 from /usr/share/mingw-w64/include/winsock2.h:23,
                 from ../../lib/setup-win32.h:91,
                 from ../../lib/curl_setup.h:308,
                 from ../../lib/idn.c:29:
/usr/share/mingw-w64/include/winnls.h:1077:30: note: previous declaration of ‘IdnToUnicode’ was here
 1077 |   WINNORMALIZEAPI int WINAPI IdnToUnicode (DWORD dwFlags, LPCWSTR lpASCIICharStr, int cchASCIIChar, LPWSTR lpUnicodeCharStr, int cchUnicodeChar);
      |                              ^~~~~~~~~~~~
```
Ref: https://github.com/curl/curl/actions/runs/10548869067/job/29223257811?pr=14680#step:7:90

This reverts commit 6c8068dfdecbef0390eac1c77e06b8db4e0fca5c.

Not good, this time with CMake, missing the original declarations:

They are guarded with in winnls.h:
```
  #if WINVER >= 0x0600
```

```
/home/runner/work/curl/curl/lib/idn.c: In function ‘win32_idn_to_ascii’:
/home/runner/work/curl/curl/lib/idn.c:174:17: error: implicit declaration of function ‘IdnToAscii’; did you mean ‘ToAscii’? [-Wimplicit-function-declaration]
  174 |     int chars = IdnToAscii(0, in_w, (int)(wcslen(in_w) + 1), punycode,
      |                 ^~~~~~~~~~
      |                 ToAscii
/home/runner/work/curl/curl/lib/idn.c:174:17: error: nested extern declaration of ‘IdnToAscii’ [-Werror=nested-externs]
/home/runner/work/curl/curl/lib/idn.c: In function ‘win32_ascii_to_idn’:
/home/runner/work/curl/curl/lib/idn.c:204:17: error: implicit declaration of function ‘IdnToUnicode’; did you mean ‘ToUnicode’? [-Wimplicit-function-declaration]
  204 |     int chars = IdnToUnicode(0, in_w, (int)(wcslen(in_w) + 1), idn,
      |                 ^~~~~~~~~~~~
      |                 ToUnicode
/home/runner/work/curl/curl/lib/idn.c:204:17: error: nested extern declaration of ‘IdnToUnicode’ [-Werror=nested-externs]
```
Ref: https://github.com/curl/curl/actions/runs/10545946413/job/29216819270?pr=13947#step:11:76

Ref: curl#12684
Ref: curl#12606 (comment)
Ref: curl@37f1c21

Originall added in 37f1c21 curl#7581
for classic mingw. curl no longer supports building with classic mingw.

Ref: curl#12684
Ref: curl#12606 (comment)