miner: restore std::cout precision after modification

Coverity 136462

src/cryptonote_basic/miner.cpp

@@ -198,7 +198,8 @@ namespace cryptonote
       {
         uint64_t total_hr = std::accumulate(m_last_hash_rates.begin(), m_last_hash_rates.end(), 0);
         float hr = static_cast<float>(total_hr)/static_cast<float>(m_last_hash_rates.size());
-        std::cout << "hashrate: " << std::setprecision(4) << std::fixed << hr << ENDL;
+        const auto precision = std::cout.precision();
+        std::cout << "hashrate: " << std::setprecision(4) << std::fixed << hr << precision << ENDL;
       }
     }
     m_last_hr_merge_time = misc_utils::get_tick_count();