Added fake landing xy velocity measurement to lpe. (#5820)

Flight tested and is working.

src/modules/local_position_estimator/BlockLocalPositionEstimator.hpp

@@ -135,7 +135,7 @@ public:
 	enum {Y_gps_x = 0, Y_gps_y, Y_gps_z, Y_gps_vx, Y_gps_vy, Y_gps_vz, n_y_gps};
 	enum {Y_vision_x = 0, Y_vision_y, Y_vision_z, n_y_vision};
 	enum {Y_mocap_x = 0, Y_mocap_y, Y_mocap_z, n_y_mocap};
-	enum {Y_land_z = 0, n_y_land};
+	enum {Y_land_vx, Y_land_vy, Y_land_agl = 0, n_y_land};
 	enum {POLL_FLOW, POLL_SENSORS, POLL_PARAM, n_poll};
 
 	BlockLocalPositionEstimator();