weston-init: fix weston not starting when xwayland is enabled

When xwayland PACKAGECONFIG option is set, xwayland is enabled in
weston.ini. However, if the xwayland module isn't installed, weston will
refuse to start with the following error message:
Failed to load module: /usr/lib/libweston-13/xwayland.so: cannot open shared object file: No such file or directory

Therefore, whenever the xwayland PACKAGECONFIG is set, weston-init
should depend on weston-xwayland to bring this module in.

Fixes: fdbe559c ("weston.init: enabled xwayland")
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>