ipv6: directly include libc-compat.h in ipv6.h

Patch 3b50d902 ("ipv6: fix redefinition of in6_pktinfo ...")
fixed a libc compatibility issue in ipv6 structure definitions
as described in include/uapi/linux/libc-compat.h.

It relies on including linux/in6.h to include libc-compat.h itself.
Include that file directly to clearly communicate the dependency
(libc-compat.h: "This include must be as early as possible").

Signed-off-by: Willem de Bruijn <willemb@google.com>

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As discussed in http://patchwork.ozlabs.org/patch/427384/


Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>