Don't reference OwnerOverlap* unless MPI is available

The class OwnerOverlapCopyCommunication is not defined unless MPI is
avilable.  Therefore, we cannot reference the type unless we know
that MPI is available in the current translation unit.

opm/core/linalg/LinearSolverIstl.cpp

@@ -225,10 +225,14 @@ namespace Opm
             break;
         case FastAMG:
 #if defined(HAS_DUNE_FAST_AMG) || DUNE_VERSION_NEWER(DUNE_ISTL, 2, 3)
+
+#if HAVE_MPI
             if(std::is_same<C,Dune::OwnerOverlapCopyCommunication<int,int> >::value)
             {
                 OPM_THROW(std::runtime_error, "Trying to use sequential FastAMG solver for a parallel problem!");
             }
+#endif // HAVE_MPI
+
             res = solveFastAMG(opA, x, b, sp, comm, linsolver_residual_tolerance_, maxit, linsolver_verbosity_,
                                linsolver_prolongate_factor_);
 #else