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: NWillem de Bruijn <willemb@google.com>

----

As discussed in http://patchwork.ozlabs.org/patch/427384/Acked-by: NCong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/uapi/linux/ipv6.h

 #ifndef _UAPI_IPV6_H
 #define _UAPI_IPV6_H
 
+#include <linux/libc-compat.h>
 #include <linux/types.h>
 #include <linux/in6.h>
 #include <asm/byteorder.h>