fix integer comparison warning

dune/vtk/localfunction.hh

@@ -101,7 +101,7 @@ namespace Dune
       double evaluate (int c, LocalCoordinate const& xi) const
       {
         assert(bool(localFct_));
-        return c < components_.size() ? localFct_->evaluate(components_[c], xi) : 0.0;
+        return c < int(components_.size()) ? localFct_->evaluate(components_[c], xi) : 0.0;
       }
 
       void setComponents (std::vector<int> components)