fixed no-mpi bug in istl backend

src/amdis/linearalgebra/istl/Communication.inc.hpp

@@ -74,6 +74,16 @@ CommunicationCreator<ISTLCommunication<Basis>>
   return std::make_unique<Communication>(basis, cat);
 }
 
-#endif
+#else // HAVE_MPI
+
+template <class Basis>
+std::unique_ptr<ISTLCommunication<Basis>>
+CommunicationCreator<ISTLCommunication<Basis>>
+  ::create(Basis const& /*basis*/, std::string const& /*prefix*/)
+{
+  return std::make_unique<Communication>();
+}
+
+#endif // HAVE_MPI
 
 } // end namespace AMDiS