xen-netfront: make bounce_skb static

The symbol is not used outside of the file, so mark it static.

Fixes the following warning:

./drivers/net/xen-netfront.c:676:16: warning: symbol 'bounce_skb' was not declared. Should it be static?
Signed-off-by: Nruanjinjie <ruanjinjie@huawei.com>
Reviewed-by: NJuergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220914064339.49841-1-ruanjinjie@huawei.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>

drivers/net/xen-netfront.c

@@ -673,7 +673,7 @@ static int xennet_xdp_xmit(struct net_device *dev, int n,
 	return nxmit;
 }
 
-struct sk_buff *bounce_skb(const struct sk_buff *skb)
+static struct sk_buff *bounce_skb(const struct sk_buff *skb)
 {
 	unsigned int headerlen = skb_headroom(skb);
 	/* Align size to allocate full pages and avoid contiguous data leaks */