do no install .so symlinks for private libraries

Summary:
since they are private and we don't install any headers there is no
point in installing the .so symlink either as it'd only be useful for
linking against

Test Plan: symlink no longer installed when running install target

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20377