explicitly request wayland-scanner feature

having qtbase include it for everyone is wasteful, so do it here
explicitly.

Change-Id: I9bf39a48fe3323aaa5471a638760ac61a61de683
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>