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