build: Build a subset of libwayland_util when not building libraries

The scanner doesn't need anything but wayland-util.c/.h so we can
shrink wayland-util when not building the main libraries.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>