[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