Use QTWEBENGINE_VERSION not QT_VERSION for WebEngine feature detection

Summary:
FreeBSD's Qt5WebEngine version is not in sync with the Qt5-version.
Therefore check the version we actually care about here.

Reviewers: #automotive, mlaurent, vkrause

Reviewed By: #automotive, vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D22300