baloo-widgets: Apply coding style to filemetadataprovider

Summary:
Prepare fixing bug 388583

CCBUG: 388583

Test Plan:
Visual inspection
make test

Reviewers: elvisangelaccio, smithjd, ngraham, vhanda, #dolphin

Reviewed By: elvisangelaccio

Differential Revision: https://phabricator.kde.org/D10143