Fix some tests not building with Qt 6

Client-side wrappers for input-method-unstable-v1 fail to build because
wl_keyboard_interface is referenced in the header file generated by
wayland-scanner.

Unfortunately, qt6_generate_wayland_protocol_client_sources() forces
--include-core-only argument, this is addressed in Qt 6.4.1, but in
meanwhile let's ship a copy of Qt6WaylandClientMacros.cmake file until
the required Qt version is out.