Recent SIMD changes break tests in dune-fem.

dune-common/dune/common/simd/interface.hh:247:72: error: invalid use of incomplete type ‘struct Dune::Simd::Overloads::IndexType<Dune::FieldMatrix<double, 1, 1>*, void>’
     using Index = typename Overloads::IndexType<std::decay_t<V> >::type;

What would be a proper fix for that?