GCS_MAVLink: expose HUD throttle accessor

libraries/GCS_MAVLink/GCS.h

@@ -635,6 +635,9 @@ public:
     
     // install an alternative protocol handler
     bool install_alternative_protocol(mavlink_channel_t chan, GCS_MAVLINK::protocol_handler_fn_t handler);
+
+    // get the VFR_HUD throttle
+    int16_t get_hud_throttle(void) const { return num_gcs()>0?chan(0).vfr_hud_throttle():0; }
     
 private: