net: make IteratorComparator's operator const

Although this is okay to be sloppy on c++11, it is not okay when
using c++17. See: https://cplusplus.github.io/LWG/issue2542

src/net_processing.cpp

@@ -43,7 +43,7 @@ std::atomic<int64_t> nTimeBestReceived(0); // Used only to inform the wallet of
 struct IteratorComparator
 {
     template<typename I>
-    bool operator()(const I& a, const I& b)
+    bool operator()(const I& a, const I& b) const
     {
         return &(*a) < &(*b);
     }