[effects/minimizeanimation] Fix coding style

Summary:
This change doesn't add new features or bugfixes, it only makes code
look uniform.

Test Plan: Still compiles.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D14608