Since `this` was captured, it does not need to be specified, but GCC 6
can't do it.  Since we want to support the standard compiler from
Debian Stretch (stable), we need to work around this problem:

 src/neighbor/plugins/UdisksNeighborPlugin.cxx:239:12: error: cannot call member function 'void UdisksNeighborExplorer::Insert(UDisks2::Object&&)' without object
       Insert(std::move(o));
       ~~~~~~^~~~~~~~~~~~~~

This fixes #300 which I previously thought was a different bug.