directory monitor: use the right 'mount_notify'

During initialisation of a directory monitor with the
G_FILE_MONITOR_WATCH_MOUNTS flag set, GLocalDirectory monitor will add a
UNIX mount watch in case the file notification backend doesn't support
reporting these events for itself.

Unfortunately, it was performing the check incorrectly, resulting in a
monitor always being added.

Fix that, and add the #define for G_LOCAL_DIRECTORY_MONITOR_GET_CLASS()
that was also missing (since the fix depends on it).

https://bugzilla.gnome.org/show_bug.cgi?id=704882