util: use gnu_printf format attribute

dtc uses non-portable formats. Using gnu_printf attributes (for
warnings) in combination with __USE_MINGW_ANSI_STDIO allows to build
for win32.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20191009102025.10179-8-marcandre.lureau@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

util.h

@@ -12,7 +12,7 @@
  */
 
 #ifdef __GNUC__
-#define PRINTF(i, j)	__attribute__((format (printf, i, j)))
+#define PRINTF(i, j)	__attribute__((format (gnu_printf, i, j)))
 #define NORETURN	__attribute__((noreturn))
 #else
 #define PRINTF(i, j)