SITL: enable updating of external payload on SIM_Plane

libraries/SITL/SIM_Plane.cpp

@@ -369,7 +369,8 @@ void Plane::update(const struct sitl_input &input)
     calculate_forces(input, rot_accel, accel_body);
     
     update_dynamics(rot_accel);
-    
+    update_external_payload(input);
+
     // update lat/lon/altitude
     update_position();
     time_advance();