tracker-miner-fs: Explicitly ignore non-native mounts

tracker-miner-fs does rely in a few places to index local content
exclusively. Make it explicitly ignore non-native mounts, they won't
be indexed anyway, but we can spare trying to create the tracker:Volume
and mount root's nfo:FileDataObject for these.

Fixes nie:url constraint errors and other warnings when trying to deal
with those.

https://bugzilla.gnome.org/show_bug.cgi?id=786132