clients/stacking: Silence a compiler warning

This patch fixes a compiler warning when building with
clang, since it doesn't support gnu_printf attribute.

v2:

 - Switch to WL_PRINTF per suggestion from Eric Engestrom.

v3:

 - Explicitly include wayland-util.h

Signed-off-by: Armin Krezović <krezovic.armin@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>

clients/stacking.c

@@ -32,6 +32,7 @@
 
 #include <linux/input.h>
 #include <cairo.h>
+#include <wayland-util.h>
 
 #include "shared/helpers.h"
 #include "window.h"
@@ -184,7 +185,7 @@ fullscreen_handler(struct window *window, void *data)
 
 static void
 draw_string(cairo_t *cr,
-            const char *fmt, ...) __attribute__((format (gnu_printf, 2, 3)));
+            const char *fmt, ...) WL_PRINTF(2, 3);
 
 static void
 draw_string(cairo_t *cr,