gunixmounts: Allow root to display mounts under /run/media/$username

Normally, the list of mounts is filtered to exclude mounts in
/run/media/$username where $username is not the current user. However,
root can access all the mounts under /run/media/, regardless of the
username — so there’s no point in filtering out those mounts.

In some cases, filtering them out is harmful. In the case of a system
service which uses GVolumeMonitor, for example, filtering them out means
the service cannot see automounted USB sticks belonging to user
sessions.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=793994
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FAIL:       2
SKIP: 1
TIMEOUT: 0

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ERROR: Job failed: exit code 1