fat-arm: Add missing define for _GNU_SOURCE

If configure finds secure_getenv it wants to use it. However it fails to
find the prototype because it is a GNU extension.

Signed-off-by: Andreas Schneider <asn@samba.org>

fat-arm.c

@@ -29,6 +29,8 @@
    not, see http://www.gnu.org/licenses/.
 */
 
+#define _GNU_SOURCE
+
 #if HAVE_CONFIG_H
 # include "config.h"
 #endif