xfrm: Make function xfrm_dev_register static

Fixes the following sparse warning:

net/xfrm/xfrm_device.c:141:5: warning:
 symbol 'xfrm_dev_register' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_device.c

@@ -138,7 +138,7 @@ ok:
 }
 EXPORT_SYMBOL_GPL(xfrm_dev_offload_ok);
 
-int xfrm_dev_register(struct net_device *dev)
+static int xfrm_dev_register(struct net_device *dev)
 {
 	if ((dev->features & NETIF_F_HW_ESP) && !dev->xfrmdev_ops)
 		return NOTIFY_BAD;