Install internal libraries with NAMELINK_SKIP

Summary:
They don't have any API/ABI guarantees, so the namelink would be mostly
useless anyway.

Test Plan: Built, .so links not installed anymore.

Reviewers: #artikulate, cordlandwehr

Reviewed By: #artikulate, cordlandwehr

Subscribers: aacid, kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D22280