Avoid comparison of unsigned int and int

dune/solvers/iterationsteps/mmgstep.cc

@@ -213,7 +213,7 @@ void MonotoneMGStep<MatrixType, VectorType>::iterate()
           for (int j=0; j<dim; j++)
             changed[i][j] = (critical[i][j] != oldCritical_[level][i][j]);
 
-        if (level < this->numLevels()-1 )
+        if (level < (int) this->numLevels()-1 )
           for (size_t i=0; i<changed.size(); i++)
             for (int j=0; j<dim; j++)
               changed[i][j] = (changed[i][j] || recompute_[level][i][j]);