[libkwineffects] Don't expose the fullscreen effect lock to the public API

Summary:
The fullscreen effect lock is purely an implementation detail of the
AnimationEffect, we don't need to have it in the public API.

Test Plan: KWin still compiles.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16075