Heli: integrate AttControl's set_yaw_target_to_current_heading

ArduCopter/heli_control_acro.pde

@@ -22,6 +22,7 @@ static void heli_acro_run()
     // unlike multicopters we do not set throttle (i.e. collective pitch) to zero so the swash servos move
     if(!motors.armed() || !motors.motor_runup_complete()) {
         attitude_control.relax_bf_rate_controller();
+        attitude_control.set_yaw_target_to_current_heading();
     }
 
     // To-Do: add support for flybarred helis