kcmkwin/scripts: fix a warning

Summary:
Prevents warnings such as :
org.kde.kcoreaddons: Expected JSON property "X-KWin-Exclude-Listing" to be a single string. but it is a bool

Reviewers: #kwin, ervin, bport, cross, hchain, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D28018