CompPair: make `operator()` a const method

This addresses the following error when using GCC 8:

```
/usr/include/c++/8/bits/stl_tree.h:457:21: error: static assertion failed: comparison object must be invocable as const
       static_assert(is_invocable_v<const _Compare&, const _Key&, const _Key&>,
```

dune/istl/matrixutils.hh

@@ -164,7 +164,7 @@ namespace Dune
   {
     struct CompPair {
       template<class G,class M>
-      bool operator()(const std::pair<G,M>& p1, const std::pair<G,M>& p2)
+      bool operator()(const std::pair<G,M>& p1, const std::pair<G,M>& p2) const
       {
         return p1.first<p2.first;
       }