Revert "Don't list KF5::WindowSystem in public libraries"

PlasmaQuick::Dialog::WindowType uses KWindowSystem's API, so removing
it from the list of the public libraries causes compilation issues in
projects that depend on plasma-framework (such as plasma-desktop) which
no longer find KWindowSystem's includes.

This reverts commit f8de13c67421e0fec216e9d471266d26be94b22e.

CCMAIL: kde@davidedmundson.co.uk