Volker Krause authored Apr 24, 2022 and Vlad Zahorodnii committed Aug 17, 2022

This helps with the Qt6 build, as  KWaylandServer::SurfaceInterface is a
QObject subclass used in a property here, while we are only able to
forward declare it here as it comes from a library we don't link against.
And that's no longer supported in Qt6.