qemu-img: Add missing GCC_FMT_ATTR

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/qemu-img.c b/qemu-img.c
index 809b4f1..f8c97d3 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -128,7 +128,7 @@
     exit(1);
 }
 
-static int qprintf(bool quiet, const char *fmt, ...)
+static int GCC_FMT_ATTR(2, 3) qprintf(bool quiet, const char *fmt, ...)
 {
     int ret = 0;
     if (!quiet) {