xen-netback: fix sparse warning

this patch fixes following sparse warning:

interface.c:83:5: warning: symbol 'xenvif_poll' was not declared. Should it be static?
Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/interface.c

@@ -80,7 +80,7 @@ static irqreturn_t xenvif_tx_interrupt(int irq, void *dev_id)
 	return IRQ_HANDLED;
 }
 
-int xenvif_poll(struct napi_struct *napi, int budget)
+static int xenvif_poll(struct napi_struct *napi, int budget)
 {
 	struct xenvif_queue *queue =
 		container_of(napi, struct xenvif_queue, napi);