Avoid duplicating screensaver interface definition file
Summary:
Kwin has a mandatory dependency on kscreenlocker, we can use the
screensaver interface definition installed from there.
Test Plan: Compiles
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D20764