build: use public-code when using the local wayland-scanner

The core wayland interfaces are public, via the libwayland-server and
libwayland-client DSOs. Hence use "public-code" cmdline option, instead
of the deprecated code".

As the host wayland-scanner may not know about the new option, use the
legacy "code".

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>