Install the PlasmaComponents.3/private folder

Summary:
It's used by ToolButton et al.
We install it in the theme folder, but not in the plasma components dir.

Test Plan: Used a toolbutton without error

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D6693