Annotate session interfaces with CAP_QUOTA

The new 'CAP_QUOTA' enum value denotes the capability quota to be
transferred from the client to the server at session-creation time.

Issue #2398

repos/os/include/timer_session/timer_session.h

@@ -28,6 +28,8 @@ struct Timer::Session : Genode::Session
 
 	static const char *service_name() { return "Timer"; }
 
+	enum { CAP_QUOTA = 2 };
+
 	virtual ~Session() { }
 
 	/**