do not install an so-symlink for elisaLib

Summary:
since the library is not public there is no reason to install the .so
symlink as it would only be used to link against the library at build time

Test Plan: builds & installs no .so symlink anymore

Reviewers: mgallien

Reviewed By: mgallien

Subscribers: Elisa

Differential Revision: https://phabricator.kde.org/D18266