Fix a compilation error with XCode Version 6.2 on OSX 10.9

src/synced_context.hpp

@@ -132,6 +132,7 @@ class synced_context
 	class server_choice
 	{
 	public:
+		virtual ~server_choice(){};
 		/// We are in a game with no mp server and need to do this choice locally
 		virtual config local_choice() const = 0;
 		/// the request which is sended to the mp server.