Propose Stefan Bruns as KFileMetaData maintainer

Summary:
He has been doing most of the work lately and seems very active, I'd say he is
at the moment the person most qualified to maintain the framework.

Test Plan:
Actually it's yellow on build.kde.org at the moment, maybe it's something to sort out
https://build.kde.org/job/Frameworks/job/kfilemetadata/

Reviewers: Frameworks, mgallien

Reviewed By: mgallien

Subscribers: mgallien, bruns, ngraham, astippich, kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20219