Copter: fixed build with optflow disabled

ArduCopter/system.pde

@@ -216,8 +216,10 @@ static void init_ardupilot()
     if(g.compass_enabled)
         init_compass();
 
+#if OPTFLOW == ENABLED
     // make optflow available to AHRS
     ahrs.set_optflow(&optflow);
+#endif
 
     // initialise attitude and position controllers
     attitude_control.set_dt(MAIN_LOOP_SECONDS);