Make xwl_output_get_xdg_output() private, it doesn't need to be
available elsewhere.
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit d31a7be15e)