Fix qmake module generation for Qt 5.12.1

Summary:
Patch by Eike Ziller from https://github.com/KDAB/GammaRay/pull/526 in
response to Qt change https://codereview.qt-project.org/#/c/248684/.

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: #frameworks, #build_system

Differential Revision: https://phabricator.kde.org/D18243