BattMonitor: fix example sketch

libraries/AP_BattMonitor/examples/AP_BattMonitor_test/AP_BattMonitor_test.pde

@@ -30,6 +30,7 @@
 #include <AP_Mission.h>
 #include <StorageManager.h>
 #include <AP_Terrain.h>
+#include <AP_RangeFinder.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;