made chat_message's operator<< inline, so there are no linker errors

chat_messages.hpp

@@ -37,7 +37,7 @@ namespace chat
         { return sizeof(char) + _content.size() + sizeof(char); }
     };
 
-    std::ostream& operator<<(std::ostream& os, const chat_message& msg)
+    inline std::ostream& operator<<(std::ostream& os, const chat_message& msg)
     { return os << msg.get(); }
 }