Allow to only build the kauth-policy-gen code generator

Summary:
This is primarily useful for cross-compilation, where e.g. with Yocto
neither Qt5Widgets nor PolicyKit are easily available for the host build.

Reviewers: Frameworks, cordlandwehr, apol

Reviewed By: apol

Subscribers: apol

Tags: Frameworks

Maniphest Tasks: T6716

Differential Revision: https://phabricator.kde.org/D7274