[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