Replace GCC_FMT_ATTR with G_GNUC_PRINTF

One less qemu-specific macro. It also helps to make some headers/units
only depend on glib, and thus moved in standalone projects eventually.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
diff --git a/qemu-img.c b/qemu-img.c
index 5dffb3e..1caddfb 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -99,7 +99,7 @@
     printf(" %s", name);
 }
 
-static void QEMU_NORETURN GCC_FMT_ATTR(1, 2) error_exit(const char *fmt, ...)
+static void QEMU_NORETURN G_GNUC_PRINTF(1, 2) error_exit(const char *fmt, ...)
 {
     va_list ap;
 
@@ -284,7 +284,7 @@
     },
 };
 
-static int GCC_FMT_ATTR(2, 3) qprintf(bool quiet, const char *fmt, ...)
+static int G_GNUC_PRINTF(2, 3) qprintf(bool quiet, const char *fmt, ...)
 {
     int ret = 0;
     if (!quiet) {