MAVLink app: Fix rate handling

src/modules/mavlink/mavlink_stream.h

@@ -86,6 +86,16 @@ public:
 	 */
 	virtual unsigned get_size() = 0;
 
+	/**
+	 * Get the average message size
+	 *
+	 * For a normal stream this equals the message size,
+	 * for something like a parameter or mission message
+	 * this equals usually zero, as no bandwidth
+	 * needs to be reserved
+	 */
+	virtual unsigned get_size_avg() { return get_size(); }
+
 protected:
 	Mavlink     *_mavlink;
 	unsigned int _interval;