util/kconfig: Silence warning about _GNU_SOURCE

For some reason, this symbol gets redefined, which causes a warning.
Hide the warning by checking whether it is already defined.

Change-Id: I70ffc9a799e0b536d6aba7d00f828bd6d915d94c
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39183
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

util/kconfig/nconf.c

@@ -5,7 +5,9 @@
  * Derived from menuconfig.
  *
  */
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 #include <string.h>
 #include <stdlib.h>