Subject private headers to headersclean [QtDeclarative]

Now that -Wweak-vtables is part of headerclean_check, we should also subject private headers to it. This should then catch all the weak vtables that matter (are accessible from other modules, causing duplication).

After QtBase has been cleaned up (QTBUG-126219), QtDeclarative as the next "core" module should be cleaned up. At that point, leaf modules can reliably opt-in to having their own private headers checked, too.