fix compilation to not require filesystem stl c++ header
Summary: Use QDir::rename on platforms where filesystem stl standard header might not be present
Test Plan: Automatic tests are OK
Reviewers: Elisa, jriddell, ngraham, pino
Reviewed By: ngraham, pino
Subscribers: pino, ngraham
Tags: Elisa
Differential Revision: https://phabricator.kde.org/D20929