!179 broke compilation because of missing backwards compatibility.

Because of !179 (merged) I have to fight compile errors just because of an arbitrary change that does not give me any added value. The MR does not work as advertised as I get

error: ‘istl’ in namespace ‘Dune::PDELab’ does not name a type
   typedef Dune::PDELab::istl::VectorBackend<> VBE0;

with current master.

I guess you have to include dune/pdelab/backend/istl.hh in every dune/pdelab/backend/istl/*.hh file