net: reference instead of copy in BlockConnected range loop

to fix -Wrange-loop-analysis warning introduced in a029e18

Author: jonatack

src/net_processing.cpp

@@ -1182,7 +1182,7 @@ void PeerLogicValidation::BlockConnected(const std::shared_ptr<const CBlock>& pb
     }
     {
         LOCK(g_cs_recent_confirmed_transactions);
-        for (const auto ptx : pblock->vtx) {
+        for (const auto& ptx : pblock->vtx) {
             g_recent_confirmed_transactions->insert(ptx->GetHash());
         }
     }