AP_BattMonitor: fixed example build

libraries/AP_BattMonitor/examples/AP_BattMonitor_test/AP_BattMonitor_test.pde

@@ -28,6 +28,7 @@
 #include <AP_NavEKF.h>
 #include <AP_Notify.h>
 #include <AP_Mission.h>
+#include <AP_Terrain.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;