GCS_MAVLink: expose HUD throttle accessor

libraries/GCS_MAVLink/GCS.h

@@ -636,6 +636,9 @@ 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:
 
     static GCS *_singleton;