xen-netback: make function xenvif_rx_skb static

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: NColin Ian King <colin.king@canonical.com>
Acked-by: NWei Liu <wei.liu2@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/rx.c

@@ -419,7 +419,7 @@ static void xenvif_rx_extra_slot(struct xenvif_queue *queue,
 	BUG();
 }
 
-void xenvif_rx_skb(struct xenvif_queue *queue)
+static void xenvif_rx_skb(struct xenvif_queue *queue)
 {
 	struct xenvif_pkt_state pkt;