build: Build libwayland-private

This splits the bulk of libwayland-util into libwayland-private.

libwayland-util (which is just wayland-util.c) is for use with the scanner.

libwayland-private is everything else.

Most things will want to link both libs.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>