AP_GPS: move defaulting of AP_GPS_UBLOX_ENABLED to AP_GPS_config

Addition of this define crossed with another PR which created the config.h file

libraries/AP_GPS/AP_GPS_UBLOX.h

@@ -20,16 +20,12 @@
 //  UBlox Lea6H protocol: http://www.u-blox.com/images/downloads/Product_Docs/u-blox6_ReceiverDescriptionProtocolSpec_%28GPS.G6-SW-10018%29.pdf
 #pragma once
 
-#include <AP_HAL/AP_HAL.h>
-
-#ifndef AP_GPS_UBLOX_ENABLED
-  #define AP_GPS_UBLOX_ENABLED 1
-#endif
+#include "AP_GPS.h"
+#include "GPS_Backend.h"
 
 #if AP_GPS_UBLOX_ENABLED
 
-#include "AP_GPS.h"
-#include "GPS_Backend.h"
+#include <AP_HAL/AP_HAL.h>
 
 /*
  *  try to put a UBlox into binary mode. This is in two parts. 

libraries/AP_GPS/AP_GPS_config.h

@@ -35,3 +35,7 @@
 #ifndef AP_GPS_SIRF_ENABLED
   #define AP_GPS_SIRF_ENABLED AP_GPS_BACKEND_DEFAULT_ENABLED
 #endif
+
+#ifndef AP_GPS_UBLOX_ENABLED
+  #define AP_GPS_UBLOX_ENABLED 1
+#endif