Fix variant visitation

So, as it turns out, pacxx supports c++17 on agamemnon, probably due to Ubuntu
18.04 (compared to Ubuntu 16.04 in the CI).  This means `Dune::Std::variant`
is just `std::variant`, rather than using Dune's fallback implementation.
Now, with the fallback implementation, it was impossible to find `visit()` via
ADL for some reason, so I had been using the member function `visit()`, which
isn't part of `std::variant`.  To satisfy both, try `using Dune::Std::visit`
before ADL.