v4l2allocator: O_CLOEXEC needs _GNU_SOURCE

Similar to 94f3d6fc / bz 709423

On some systems (E.G. uClibc and older Glibc versions), O_CLOEXEC is only
defined when _GNU_SOURCE is specified, so do so.

https://bugzilla.gnome.org/show_bug.cgi?id=736670

sys/v4l2/gstv4l2allocator.c

@@ -21,6 +21,10 @@
 
 #include "config.h"
 
+#ifndef _GNU_SOURCE
+# define _GNU_SOURCE            /* O_CLOEXEC */
+#endif
+
 #include "ext/videodev2.h"
 #include "gstv4l2allocator.h"
 #include "v4l2_calls.h"