wayland-shm: Include wayland-util.h

wayland-shm.c uses WL_EXPORT and wl_array, which are defined in
wayland-util.h.

Include wayland-util.h explicitly, rather than transitively through
wayland-server.h.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Sam Spilsbury <smspillaz@gmail.com>