KF5NewStuff is only used in moleculeview, conditional on Eigen3 and AvogadroLibs

Test Plan: Built fine with AvogadroLibs disabled. Unfortunately no testing if enabled, but should be fine.

Reviewers: #kde_edu, apol

Reviewed By: apol

Subscribers: #kde_edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D9920