Allow Kirigami build without KWin tablet mode dependency

Summary:
Even on linux it might be the case to not need/want the tablet mode manager dependency.

This is the case for Subsurface-mobile, where desktop builds (linux and Mac) are used for development, with the final target being iOS and Android.

Reviewers: mart, davidedmundson

Reviewed By: mart

Subscribers: apol, davidedmundson, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D13148