[KCM] Use a better icon

Summary: Icon was already there, so we can do this in 5.15.

Test Plan: {F6565234, size=full}

Reviewers: #vdg, #plasma, ndavis

Reviewed By: #vdg, ndavis

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18491