Fix build after 0b2fe3cf21
Summary: QVector is not unused in kstringhandler.cpp
Test Plan: kcoreaddons master builds against Qt 5.12.4
Reviewers: dfaure, broulik
Reviewed By: broulik
Subscribers: broulik, kde-frameworks-devel, mlaurent
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D22413