Fix cmake test_moc_macro_target with namespace

The interface macro needs to be in the Qt namespace, otherwise the
template specialization in Q_DECLARE_INTERFACE fails.

Task-number: QTBUG-41959
Change-Id: I818da5ba8460219d73c0d248ee410ac92f7498f4
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>
Reviewed-by: Stephen Kelly <steveire@gmail.com>