vxlan: include net/ip6_checksum.h for csum_ipv6_magic()

Fengguang reported a compile warning:

   drivers/net/vxlan.c: In function 'vxlan6_xmit_skb':
   drivers/net/vxlan.c:1352:3: error: implicit declaration of function 'csum_ipv6_magic' [-Werror=implicit-function-declaration]
   cc1: some warnings being treated as errors

this patch fixes it.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>