[FirstRunIndexer] Use correct mimetype for folders

Summary:
mimeTypeForFile yields an incorrect mimetype for folders, just hardcode
"inode/directory".

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: poboiko

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21704