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