qemu-img: Add missing GCC_FMT_ATTR

Signed-off-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qemu-img.c

@@ -128,7 +128,7 @@ static void help(void)
     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) {