Set transfer operator also when HAVE_MPI is set

This is a fall-out from the recent move to use all the parallel code
only when HAVE_MPI is set.  Very ugly, but I still don't know any other
way to have P3 spaces.

Patch by Jonathan Youett

[[Imported from SVN: r10054]]

dune/gfe/riemanniantrsolver.cc

@@ -270,6 +270,7 @@ setup(const GridType& grid,
                            LevelLocalMapper> matrixComm(fineGUIndex, coarseGUIndex, grid_->levelGridView(i+1), grid_->levelGridView(i), fineLevelLocalMapper, coarseLevelLocalMapper, 0);
 
         mmgStep->mgTransfer_[i] = new TruncatedCompressedMGTransfer<CorrectionType>;
+        std::shared_ptr<TransferOperatorType> transferOperatorMatrix = std::make_shared<TransferOperatorType>(matrixComm.reduceCopy(newTransferOp->getMatrix()));
 
 #else
         mmgStep->mgTransfer_[i] = new TruncatedCompressedMGTransfer<CorrectionType>;