Fix dangling reference with "auto" becoming "QStringBuilder"

Summary:
With QT_USE_QSTRINGBUILDER being enabled, code best is checked with
clazy -checks=auto-unexpected-qstringbuilder for these traps now enabled

Reviewers: ivan, mlaurent, davidedmundson

Reviewed By: davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15513