QAtomicInteger::loadRelaxed() and storeRelaxed() are available since Qt 5.14

Test Plan: make && ctest

Reviewers: Baloo, meven, bruns, astippich, mlaurent

Reviewed By: mlaurent

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D26102