Fix -Wimplicit-function-declaration errors with musl and gcc 14.

```
In file included from ../desmume/src/libretro-common/include/rthreads/rthreads.h:31,
                 from ../desmume/src/libretro-common/rthreads/async_job.c:25:
../desmume/src/libretro-common/include/retro_miscellaneous.h: In function 'retro_sleep':
../desmume/src/libretro-common/include/retro_miscellaneous.h:107:4: error: implicit declaration of function 'nanosleep' [-Wimplicit-func
tion-declaration]
  107 |    nanosleep(&tv, NULL);
      |    ^~~~~~~~~
```

```
../desmume/src/libretro-common/file/file_path.c: In function 'path_mkdir':
../desmume/src/libretro-common/file/file_path.c:53:24: error: implicit declaration of function 'strdup'; did you mean 'strcmp'? [-Wimpli
cit-function-declaration]
   53 |    char     *basedir = strdup(dir);
      |                        ^~~~~~
      |                        strcmp
../desmume/src/libretro-common/file/file_path.c:53:24: error: initialization of 'char *' from 'int' makes pointer from integer without a
 cast [-Wint-conversion]
../desmume/src/libretro-common/file/file_path.c: In function 'path_resolve_realpath':
../desmume/src/libretro-common/file/file_path.c:541:9: error: implicit declaration of function 'realpath' [-Wimplicit-function-declarati
on]
  541 |    if (!realpath(tmp, buf))
      |         ^~~~~~~~
```

--- desmume-0.9.13-origin/desmume/src/libretro-common/file/file_path.c
+++ desmume-0.9.13/desmume/src/libretro-common/file/file_path.c
@@ -20,8 +20,16 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
 
+/* stdlib.h realpath() */
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
 #include <stdlib.h>
 #include <boolean.h>
+/* string.h strdup() */
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 199309L
+#endif
 #include <string.h>
 #include <time.h>
 #include <errno.h>
--- desmume-0.9.13-origin/desmume/src/libretro-common/include/retro_miscellaneous.h
+++ desmume-0.9.13/desmume/src/libretro-common/include/retro_miscellaneous.h
@@ -39,6 +39,10 @@
 #elif defined(_3DS)
 #include <3ds.h>
 #else
+/* time.h nanosleep() */
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 199309L
+#endif
 #include <time.h>
 #endif