Add mutex round static member used in a thread

Summary:
BackgroundFinder runs in a separate thread, it uses a static QStringList cache
It's perfectly plausible that two BackgroundFinders threads could run at once

Test Plan:
Old code didn't crash, it was a pure hypothetical.
Works as before

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: anthonyfieroni, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11590