SITL: helicopter add update_external_payload() for sensors

libraries/SITL/SIM_Helicopter.cpp

@@ -316,6 +316,7 @@ void Helicopter::update(const struct sitl_input &input)
 
 
     update_dynamics(rot_accel);
+    update_external_payload(input);
 
     // update lat/lon/altitude
     update_position();