ixp4xx_eth: fix ptp_ixp46x build failure

When building with ixp4xx_eth and ptp_ixp46x as module, one is getting the
following error:

ERROR: "ixp46x_phc_index" [drivers/ptp/ptp_ixp46x.ko] undefined!

This has been introduced by commit 509a7c25.
Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/xscale/ixp4xx_eth.c

@@ -1003,6 +1003,7 @@ static int ixp4xx_nway_reset(struct net_device *dev)
 }
 
 int ixp46x_phc_index = -1;
+EXPORT_SYMBOL_GPL(ixp46x_phc_index);
 
 static int ixp4xx_get_ts_info(struct net_device *dev,
 			      struct ethtool_ts_info *info)