AP_Motors: fix example

libraries/AP_Motors/examples/AP_Motors_test/AP_Motors_test.cpp

@@ -22,6 +22,7 @@
 #include <AP_Declination/AP_Declination.h>
 #include <AP_Airspeed/AP_Airspeed.h>
 #include <AP_Vehicle/AP_Vehicle.h>
+#include <AP_BattMonitor/AP_BattMonitor.h>
 #include <AP_Mission/AP_Mission.h>
 #include <StorageManager/StorageManager.h>
 #include <AP_Terrain/AP_Terrain.h>
@@ -52,6 +53,8 @@ AP_MotorsMatrix   motors(400);
 //AP_MotorsSingle motors(400);
 //AP_MotorsCoax motors(400);
 
+AP_BattMonitor _battmonitor{0, nullptr, nullptr};
+
 // setup
 void setup()
 {