add .load() to make Boost 1.67 happy with its new is_integral check

src/wallet/api/wallet.cpp

@@ -1740,7 +1740,7 @@ void WalletImpl::refreshThreadFunc()
         // if auto refresh enabled, we wait for the "m_refreshIntervalSeconds" interval.
         // if not - we wait forever
         if (m_refreshIntervalMillis > 0) {
-            boost::posix_time::milliseconds wait_for_ms(m_refreshIntervalMillis);
+            boost::posix_time::milliseconds wait_for_ms(m_refreshIntervalMillis.load());
             m_refreshCV.timed_wait(lock, wait_for_ms);
         } else {
             m_refreshCV.wait(lock);