[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