AP_Beacon: fix example when Beacon not available

simply print the fact that the beacon librar y isn't compiled in

libraries/AP_Beacon/examples/AP_Marvelmind_test/AP_Marvelmind_test.cpp

@@ -16,7 +16,9 @@ void set_object_value_and_report(const void *object_pointer,
 
 const AP_HAL::HAL& hal = AP_HAL::get_HAL();
 static AP_SerialManager serial_manager;
+#if AP_BEACON_ENABLED
 AP_Beacon beacon;
+#endif
 
 // try to set the object value but provide diagnostic if it failed
 void set_object_value_and_report(const void *object_pointer,
@@ -31,14 +33,19 @@ void set_object_value_and_report(const void *object_pointer,
 
 void setup(void)
 {
+#if AP_BEACON_ENABLED
     set_object_value_and_report(&beacon, beacon.var_info, "_TYPE", 2.0f);
     set_object_value_and_report(&serial_manager, serial_manager.var_info, "0_PROTOCOL", 13.0f);
+#endif
     serial_manager.init();
+#if AP_BEACON_ENABLED
     beacon.init();
+#endif
 }
 
 void loop(void)
 {
+#if AP_BEACON_ENABLED
     static int count = 0;
     beacon.update();
     Vector3f pos;
@@ -51,6 +58,10 @@ void loop(void)
     hal.scheduler->delay(1000);
     if (count == 3)
         exit(0);
+#else
+    printf("Beacon not available\n");
+    hal.scheduler->delay(1000);
+#endif
 }
 
 AP_HAL_MAIN();