AP_Math: fix example sketch

libraries/AP_Math/examples/eulers/eulers.pde

@@ -34,6 +34,7 @@
 #include <AP_Vehicle.h>
 #include <AP_ADC_AnalogSource.h>
 #include <AP_Rally.h>
+#include <AP_BattMonitor.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;
 

libraries/AP_Math/examples/location/location.pde

@@ -32,6 +32,7 @@
 #include <AP_Vehicle.h>
 #include <AP_ADC_AnalogSource.h>
 #include <AP_Compass.h>
+#include <AP_BattMonitor.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;
 

libraries/AP_Math/examples/rotations/rotations.pde

@@ -33,6 +33,7 @@
 #include <AP_Vehicle.h>
 #include <AP_ADC_AnalogSource.h>
 #include <AP_Rally.h>
+#include <AP_BattMonitor.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;