xen-netback: include definition of csum_ipv6_magic

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: NAndy Whitcroft <apw@canonical.com>
Acked-by: NIan Campbell <ian.campbell@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -39,6 +39,7 @@
 #include <linux/udp.h>
 
 #include <net/tcp.h>
+#include <net/ip6_checksum.h>
 
 #include <xen/xen.h>
 #include <xen/events.h>