ip6tnl: Use <netinet/in.h> for in6_addr and stuff

<linux/in6.h> is not compatible with some versions of <netinet/in.h>.

Cc: Susant Sahani <susant@redhat.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>