The function xenvif_rx_skb is local to the source and does not need
to be in global scope, so make it static.

Cleans up sparse warning:
drivers/net/xen-netback/rx.c:422:6: warning: symbol 'xenvif_rx_skb'
was not declared. Should it be static?
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>