[FileMetaDataProvider] Avoid detach of m_fileItems

Summary: Use qAsConst to avoid detach.

Test Plan: no functional changes

Reviewers: Baloo, Dolphin, nicolasfella, astippich, ngraham, elvisangelaccio, broulik

Reviewed By: broulik

Tags: Baloo

Differential Revision: https://phabricator.kde.org/D20911