Qt Wayland doesn't build with old versions of libwayland

We recently upgraded wayland.xml to 1.10, although we're supposed to support libwayland as old as 1.6

This should have been fine, however having a newer version of wayland.xml than libwayland is only supported if only the core includes are used. I.e. wayland-client-core instead of wayland-client. Ditto for server.

Consequently, libwayland doesn't currently build with libwayland 1.9 or older.