From: Andy Whitcroft <apw@canonical.com>
Date: Mon, 25 Nov 2013 16:52:34 +0000

We are now using csum_ipv6_magic, include the appropriate header.
Avoids the following error:

    drivers/net/xen-netback/netback.c:1313:4: error: implicit declaration of function 'csum_ipv6_magic' [-Werror=implicit-function-declaration]
        tcph->check = ~csum_ipv6_magic(&ipv6h->saddr,

Signed-off-by: Andy Whitcroft <apw@canonical.com>

Applied, thank you.