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: ruanjinjie <ruanjinjie@huawei.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220914064339.49841-1-ruanjinjie@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>