Summary:
* effects.h includes client.h and forward declares Client, both at the
  same time. Thus, delete the include;

* the blur effect includes effects.h. That, most likely, is a leftover
  after 3f5bf65a.

Test Plan: Compiles.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15191