Simplify cmake code: link to Qt5::GuiPrivate.

Summary:
This target was added in Qt 5.9, which we can now rely upon.
Also use target_include_directories rather than the big
include_directories hammer.

Test Plan: Compiles

Reviewers: kfunk, kossebau, broulik

Reviewed By: kfunk, broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15429