tools/Makefile: Move _GNU_SOURCE to Makefile

Commit 669dfc2e adds libfdt_env.h to HOSTCPPFLAGS. It causes stdio.h
to be included before _GNU_SOURCE is defined in C files. On some old hosts
some prototypes are protected by #ifdef __USE_GNU, which is set when
_GNU_SOURCE is defined.

Signed-off-by: York Sun <yorksun@freescale.com>
Acked-by: Simon Glass <sjg@chromium.org>

tools/imximage.c

@@ -9,9 +9,6 @@
  * SPDX-License-Identifier:	GPL-2.0+ 
  */
 
-/* Required to obtain the getline prototype from stdio.h */
-#define _GNU_SOURCE
-
 #include "mkimage.h"
 #include <image.h>
 #include "imximage.h"