Do not leak symbols of pimpl classes, protect with Q_DECL_HIDDEN
Summary:
Nested classes inherit the visibility of their outer class,
so all the KClass::Private symbols have also been exported
if the ones of KClass were
Reviewers: graesslin, davidedmundson
Reviewed By: davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D11209