IPv6: Fix 6RD build error

Fix build error introduced in commit fa857afc - ipv6 sit: 6rd
(IPv6 Rapid Deployment) Support.  Struct in6_addr is the issue.
I'm only seeing this on x86_64 systems, not on 32-bit with same
IPv6 config options, so it could be there's a missing forward
declaration somewhere, but including the correct header file
fixes the problem too.

  CC [M]  net/ipv6/ip6_tunnel.o
In file included from net/ipv6/ip6_tunnel.c:31:
include/linux/if_tunnel.h:59: error: field ‘prefix’ has incomplete type
make[2]: *** [net/ipv6/ip6_tunnel.o] Error 1
make[1]: *** [net/ipv6] Error 2

Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>