server: use the new wl_priv_signal for wl_resource

The old wl_signal is kept for backwards compatibility, as that is also
present in the deprecated public wl_resource struct, and that must be
kept working.

Signed-off-by: Giulio Camuffo <giulio.camuffo@kdab.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>