community/phosh: add xwayland to depends

phosh session file currently requires org.gnome.SettingsDaemon.XSettings as a required component. Such component needs XWayland when running under wayland to be able to start.

In debian they solve it adding the XWayland dependency to gnome-session. However, I don't think this is the right approach since gnome-session picks the set of required components from the session file at runtime.