Add includes as suggested to fix missing implicit declarations and a
bunch of build failures that appeared with gcc 14:

```
cc -O2 -g -Wdate-time -D_FORTIFY_SOURCE=2 -Wl,-z,relro  splitfont.c   -o splitfont
splitfont.c: In function ‘dosplit’:
splitfont.c:19:17: error: implicit declaration of function ‘exit’ [-Wimplicit-function-declaration]
   19 |                 exit(1);
      |                 ^~~~
splitfont.c:8:1: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
    7 | #include <sys/stat.h>
  +++ |+#include <stdlib.h>
    8 |
splitfont.c:19:17: warning: incompatible implicit declaration of built-in function ‘exit’ [-Wbuiltin-declaration-mismatch]
   19 |                 exit(1);
      |                 ^~~~
splitfont.c:19:17: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
splitfont.c:21:13: error: implicit declaration of function ‘strlen’ [-Wimplicit-function-declaration]
   21 |         if (strlen(fontfile) >= sizeof(filename) - 4) {
      |             ^~~~~~
splitfont.c:8:1: note: include ‘<string.h>’ or provide a declaration of ‘strlen’
    7 | #include <sys/stat.h>
  +++ |+#include <string.h>
    8 |
splitfont.c:21:13: warning: incompatible implicit declaration of built-in function ‘strlen’ [-Wbuiltin-declaration-mismatch]
   21 |         if (strlen(fontfile) >= sizeof(filename) - 4) {
      |             ^~~~~~
splitfont.c:21:13: note: include ‘<string.h>’ or provide a declaration of ‘strlen’
splitfont.c:23:17: warning: incompatible implicit declaration of built-in function ‘exit’ [-Wbuiltin-declaration-mismatch]
   23 |                 exit(1);
      |                 ^~~~
splitfont.c:23:17: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
splitfont.c: In function ‘main’:
splitfont.c:51:17: warning: incompatible implicit declaration of built-in function ‘exit’ [-Wbuiltin-declaration-mismatch]
   51 |                 exit(1);
      |                 ^~~~
splitfont.c:51:17: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
splitfont.c:56:17: warning: incompatible implicit declaration of built-in function ‘exit’ [-Wbuiltin-declaration-mismatch]
   56 |                 exit(1);
      |                 ^~~~
splitfont.c:56:17: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
splitfont.c:60:17: warning: incompatible implicit declaration of built-in function ‘exit’ [-Wbuiltin-declaration-mismatch]
   60 |                 exit(1);
      |                 ^~~~
splitfont.c:60:17: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
splitfont.c:64:17: warning: incompatible implicit declaration of built-in function ‘exit’ [-Wbuiltin-declaration-mismatch]
   64 |                 exit(1);
      |                 ^~~~
splitfont.c:64:17: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
splitfont.c:69:17: warning: incompatible implicit declaration of built-in function ‘exit’ [-Wbuiltin-declaration-mismatch]
   69 |                 exit(1);
      |                 ^~~~
splitfont.c:69:17: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
splitfont.c:74:17: warning: incompatible implicit declaration of built-in function ‘exit’ [-Wbuiltin-declaration-mismatch]
   74 |                 exit(1);
      |                 ^~~~
splitfont.c:74:17: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
splitfont.c:79:29: error: implicit declaration of function ‘strtoul’ [-Wimplicit-function-declaration]
   79 |                 to = from = strtoul(p, &q, 0);
      |                             ^~~~~~~
splitfont.c:86:25: warning: incompatible implicit declaration of built-in function ‘exit’ [-Wbuiltin-declaration-mismatch]
   86 |                         exit(1);
      |                         ^~~~
splitfont.c:86:25: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
```

Originally reported at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1075109

Signed-off-by: Andreas Henriksson <andreas@fatal.se>