block/qcow2: Add missing GCC_FMT_ATTR to function report_unsupported()

Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -182,7 +182,8 @@ static void cleanup_unknown_header_ext(BlockDriverState *bs)
     }
 }
 
-static void report_unsupported(BlockDriverState *bs, const char *fmt, ...)
+static void GCC_FMT_ATTR(2, 3) report_unsupported(BlockDriverState *bs,
+    const char *fmt, ...)
 {
     char msg[64];
     va_list ap;