Fix compiler warning in imximage.c due to getline prototype

imximage.c: In function 'imximage_parse_cfg_file':
imximage.c:142: warning: implicit declaration of function 'getline'

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

tools/imximage.c

@@ -25,6 +25,9 @@
  * MA 02111-1307 USA
  */
 
+/* Required to obtain the getline prototype from stdio.h */
+#define _GNU_SOURCE
+
 #include "mkimage.h"
 #include <image.h>
 #include "imximage.h"