Add KF5WindowSystem to link interface

Summary:
Otherwise, reverse-dependencies are bugged
with finding KF5WindowSystem themselves.

Test Plan: Builds fine, plasma-sdk does not need to add KF5WindowSystem anymore.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel, davidedmundson, Frameworks

Tags: Frameworks, Plasma

Differential Revision: https://phabricator.kde.org/D6493