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