AP_Vehicle: correct compilation when AP_Vehicle not available

libraries/AP_Vehicle/AP_Vehicle.cpp

@@ -1,3 +1,7 @@
+#include "AP_Vehicle_config.h"
+
+#if AP_VEHICLE_ENABLED
+
 #include "AP_Vehicle.h"
 
 #include <AP_BLHeli/AP_BLHeli.h>
@@ -1029,3 +1033,4 @@ AP_Vehicle *vehicle()
 
 };
 
+#endif  // AP_VEHICLE_ENABLED

libraries/AP_Vehicle/AP_Vehicle.h

@@ -14,6 +14,10 @@
  */
 #pragma once
 
+#include "AP_Vehicle_config.h"
+
+#if AP_VEHICLE_ENABLED
+
 /*
   this header holds a parameter structure for each vehicle type for
   parameters needed by multiple libraries
@@ -490,3 +494,5 @@ extern const AP_HAL::HAL& hal;
 extern const AP_Param::Info vehicle_var_info[];
 
 #include "AP_Vehicle_Type.h"
+
+#endif  // AP_VEHICLE_ENABLED

libraries/AP_Vehicle/AP_Vehicle_config.h

@@ -0,0 +1,5 @@
+#pragma once
+
+#ifndef AP_VEHICLE_ENABLED
+#define AP_VEHICLE_ENABLED 1
+#endif