AC_WPNAV: fixed example build

libraries/AC_WPNav/examples/AC_WPNav_test/AC_WPNav_test.pde

@@ -25,6 +25,7 @@
 #include <AP_Buffer.h>          // ArduPilot general purpose FIFO buffer
 #include <AP_InertialNav.h>     // Inertial Navigation library
 #include <AC_WPNav.h>           // Waypoint Navigation library
+#include <DataFlash.h>
 #include <GCS_MAVLink.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;