event-loop: Include wayland-util.h

event-loop.c uses WL_EXPORT and wl_list, which are defined in
wayland-util.h.

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

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