AP_Airspeed: correct example; need instantiated AHRS for windspeed_max

libraries/AP_Airspeed/examples/Airspeed/Airspeed.cpp

@@ -18,6 +18,7 @@
  *
  */
 
+#include <AP_AHRS/AP_AHRS.h>
 #include <AP_Airspeed/AP_Airspeed.h>
 #include <AP_HAL/AP_HAL.h>
 #include <AP_BoardConfig/AP_BoardConfig.h>
@@ -30,6 +31,9 @@ const AP_HAL::HAL& hal = AP_HAL::get_HAL();
 
 float temperature;
 
+// create an AHRS object for get_airspeed_max
+AP_AHRS_DCM ahrs;
+
 // create airspeed object
 AP_Airspeed airspeed;