[plugins/windowsystem] Fix compilation with no-deprecated build of KWindowSystem
Summary:
This change ensures that KWin can be built with a version of KWS that
doesn't have any deprecated stuff. I assume that no distro ships such
builds of KWS and it's only a developer thing.
I didn't backport KWINDOWSYSTEM_BUILD_DEPRECATED_SINCE macro because
the next release of Plasma (5.18) depends on KF 5.66.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D26642