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>
1 file changed