AP_GPS: fixed example build

libraries/AP_GPS/examples/GPS_AUTO_test/GPS_AUTO_test.pde

@@ -30,6 +30,7 @@
 #include <AP_Math.h>
 #include <AP_Notify.h>
 #include <AP_BoardLED.h>
+#include <AP_NavEKF.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;