cryptonote: Add const-qualifier on comparison functor

The original code did not compile with GCC 8.2.1 in C++17 mode, since
comparison functions for std::set's must be invocable as const.

src/cryptonote_core/tx_pool.h

@@ -61,7 +61,7 @@ namespace cryptonote
   class txCompare
   {
   public:
-    bool operator()(const tx_by_fee_and_receive_time_entry& a, const tx_by_fee_and_receive_time_entry& b)
+    bool operator()(const tx_by_fee_and_receive_time_entry& a, const tx_by_fee_and_receive_time_entry& b) const
     {
       // sort by greatest first, not least
       if (a.first.first > b.first.first) return true;