Add missing namespace fixes for addToDiagonal

Apparently the calls are not used anywhere in dune-solvers
itself, that's why the CI system didn't catch the problem.

dune/solvers/test/common.hh

@@ -189,11 +189,11 @@ void assemblePQ1Mass(const GridView& gridView, Matrix& matrix)
                     int jGlobal = indexSet.subIndex(element, fe.localCoefficients().localKey(j).subEntity(), dim);
 
                     double zij = values[j] * zi;
-                    addToDiagonal(matrix[iGlobal][jGlobal], zij);
-                    addToDiagonal(matrix[jGlobal][iGlobal], zij);
+                    Dune::MatrixVector::addToDiagonal(matrix[iGlobal][jGlobal], zij);
+                    Dune::MatrixVector::addToDiagonal(matrix[jGlobal][iGlobal], zij);
                 }
                 double zii = values[i] * zi;
-                addToDiagonal(matrix[iGlobal][iGlobal], zii);
+                Dune::MatrixVector::addToDiagonal(matrix[iGlobal][iGlobal], zii);
             }
         }
     }