Move a redundant _GNU_SOURCE to where it is not redundant

src/common/compat_pthreads.c

@@ -3,6 +3,8 @@
  * Copyright (c) 2007-2015, The Tor Project, Inc. */
 /* See LICENSE for licensing information */
 
+#define _GNU_SOURCE
+
 #include "orconfig.h"
 #include <pthread.h>
 #include <signal.h>

src/common/compat_threads.c

@@ -6,7 +6,6 @@
 #define _GNU_SOURCE
 
 #include "orconfig.h"
-#define _GNU_SOURCE
 #include <stdlib.h>
 #include "compat.h"
 #include "compat_threads.h"