libretro-common: fix implicit declarations

strdup and realpath are only declared by glibc's headers if
_XOPEN_SOURCE >= 500.

desmume/src/libretro-common/file/file_path.c

@@ -20,6 +20,8 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#define _XOPEN_SOURCE 500   /* For strdup, realpath */
+
 #include <stdlib.h>
 #include <boolean.h>
 #include <string.h>