AHRS: fix example sketch

libraries/AP_AHRS/examples/AHRS_Test/AHRS_Test.pde

@@ -37,6 +37,7 @@
 #include <AP_HAL_AVR_SITL.h>
 #include <AP_HAL_Empty.h>
 #include <AP_HAL_PX4.h>
+#include <AP_BattMonitor.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;