add trivial systemsettings finder and mark it as RUNTIME dep

Summary:
simply finds systemsettings5 in the expected path and report it as part
of the feature summary so everyone makes sure systemsettings is actually
installed

Test Plan: reported missing/present as applicable

Reviewers: mart, broulik, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27026