Copter: fixed build with specified GPS driver

ArduCopter/ArduCopter.pde

@@ -240,22 +240,22 @@ static AP_Compass_HMC5843 compass;
 AP_GPS_Auto     g_gps_driver(&g_gps);
 
  #elif GPS_PROTOCOL == GPS_PROTOCOL_NMEA
-AP_GPS_NMEA     g_gps_driver();
+AP_GPS_NMEA     g_gps_driver;
 
  #elif GPS_PROTOCOL == GPS_PROTOCOL_SIRF
-AP_GPS_SIRF     g_gps_driver();
+AP_GPS_SIRF     g_gps_driver;
 
  #elif GPS_PROTOCOL == GPS_PROTOCOL_UBLOX
-AP_GPS_UBLOX    g_gps_driver();
+AP_GPS_UBLOX    g_gps_driver;
 
  #elif GPS_PROTOCOL == GPS_PROTOCOL_MTK
-AP_GPS_MTK      g_gps_driver();
+AP_GPS_MTK      g_gps_driver;
 
  #elif GPS_PROTOCOL == GPS_PROTOCOL_MTK19
-AP_GPS_MTK19    g_gps_driver();
+AP_GPS_MTK19    g_gps_driver;
 
  #elif GPS_PROTOCOL == GPS_PROTOCOL_NONE
-AP_GPS_None     g_gps_driver();
+AP_GPS_None     g_gps_driver;
 
  #else
   #error Unrecognised GPS_PROTOCOL setting.