Plane: make optflow available to AHRS

ArduPlane/system.pde

@@ -168,6 +168,9 @@ static void init_ardupilot()
         }
     }
     
+    // make optflow available to libraries
+    ahrs.set_optflow(&optflow);
+
     // Register mavlink_delay_cb, which will run anytime you have
     // more than 5ms remaining in your call to hal.scheduler->delay
     hal.scheduler->register_delay_callback(mavlink_delay_cb, 5);