wayland-util.h: add forward declaration for wl_object

The definition of wl_argument in wayland-util.h references wl_object,
so wl_object ought to be defined in wayland-util.h. This resolves
gitlab issue #78.

Fixes: https://gitlab.freedesktop.org/wayland/wayland/issues/78



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>