wayland-server: Clarify included header dependencies

wayland-server.c directly depends on wayland-util.h, and will include
wayland-server-protocol.h via wayland-server.h.

Explicitly include wayland-util.h, making this dependency clear.
Remove the redundant inclusion of wayland-server-protocol.h.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>