[kcmkwin/compositing] Use new icon for Effects KCM

Summary: Use the new icon introduced in D18457

Test Plan:

Reviewers: VDG, davidedmundson, GB_2

Reviewed By: VDG, davidedmundson, GB_2

Subscribers: Codezela, GB_2, zzag, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18461