[kcmkwin/kwindecoration] Add missing QT include

Summary: Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14173