[effects/slide] Fix coding style

Summary:
Kdelibs coding style states the following about whitespace placement:

> For pointers or references, use a single space before '*' or '&', but not after

Source: https://community.kde.org/Policies/Kdelibs_Coding_Style#Whitespace

Also, putting a whitespace after ! is not common in KWin.

Test Plan: Switched between virtual desktops.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14260