Fix build due to missing QVector include
Summary:
I get 'implicit instantiation of undefined template 'QVector<QString>'
when building against Qt 5.12.4 without this change.
Test Plan: Builds against Qt 5.12.4
Reviewers: dfaure, davidedmundson
Reviewed By: davidedmundson
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D22414