meson: fix compositor build with xwayland disabled

If xwayland is disabled, compositor/weston is built without
compositor/xwayland.c, which defines wet_load_xwayland.

  compositor/fb12c4d@@Weston@exe/main.c.o: In function `main':
  ../weston-5.0.0-169-g2d4cc4f4dd6a/compositor/main.c:3103: undefined reference to `wet_load_xwayland'

Provide an empty stub for wet_load_xwayland if xwayland is disabled.
With that we also have to remove xwayland.c from the autotools build
if xwayland is disabled, to avoid a multiple definition error.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>