Properly fix copy constructor

Summary:
Instead of printing a warning (which probably nobody would notice) just
fix the code, i.e. copy the private.

Test Plan: ctest

Reviewers: Baloo, Frameworks, ngraham, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D19961