Add RTC_CPP_EXPORT before to_variant function

include/rtc/message.hpp

@@ -74,7 +74,7 @@ RTC_CPP_EXPORT message_ptr make_message(binary &&data, Message::Type type = Mess
 
 RTC_CPP_EXPORT message_ptr make_message(message_variant data);
 
-message_variant to_variant(Message &&message);
+RTC_CPP_EXPORT message_variant to_variant(Message &&message);
 
 } // namespace rtc