Explicitly link against KAuth

Summary: Otherwise it fails on Android, probably because the KAuth dependency was removed there conditionally for kconfigwidgets.

Reviewers: Kate, Frameworks, vkrause, cullmann

Reviewed By: Kate, cullmann

Subscribers: cullmann, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18995