Wallet: Fix unused variable compiler warning

g++ 7.4.0 on Ubuntu 18.04

src/wallet/wallet2.cpp

@@ -3542,6 +3542,7 @@ void wallet2::refresh(bool trusted_daemon, uint64_t start_height, uint64_t & blo
   std::vector<cryptonote::block_complete_entry> blocks;
   std::vector<parsed_block> parsed_blocks;
   bool refreshed = false;
+  (void)refreshed;  // Silence a compiler warning.
   std::shared_ptr<std::map<std::pair<uint64_t, uint64_t>, size_t>> output_tracker_cache;
   hw::device &hwdev = m_account.get_device();