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