platform/posix: fix _GNU_SOURCE define in lib_posix

engine/platform/posix/lib_posix.c

@@ -12,9 +12,11 @@ but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 GNU General Public License for more details.
 */
+
+#define _GNU_SOURCE
 #include "platform/platform.h"
 #if XASH_LIB == LIB_POSIX
-#define _GNU_SOURCE
+
 
 #include "common.h"
 #include "library.h"