xwayland: Move includes to avoid deprecation warnings

xwayland: Move includes to avoid deprecation warnings

compositor.h already helpfully defines WL_HIDE_DEPRECATED for us, so we
don't get warnings about wl_buffer (in particular) being deprecated when
we have wayland-server headers defining it as deprecated, and then
wayland-client headers using the type.

Move it to be before all our other includes, so we actually make use of
it.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Tested-by: Yong Bakos <ybakos@humanoriented.com>