Fix build with a KWindowSystem framework that doesn't pull QWidget
Summary: Fixes the build with D8705
Test Plan: Just adds includes, I wonder if it should go into Plasma/5.8 and /5.11 too
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: graesslin, plasma-devel, kwin, KWin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D8706