Add TCP_REPAIR_WINDOW from Linux 4.8

[Florian Weimer]

* Joseph Myers:

> Linux 4.8 adds TCP_REPAIR_WINDOW to include/uapi/linux/tcp.h.  This
> patch adds it to sysdeps/gnu/netinet/tcp.h accordingly.
>
> Tested for x86_64 and x86 (testsuite, and that installed shared
> libraries are unchanged by the patch).

Please also add struct tcp_repair_window, otherwise the #define
doesn't make much sense.  Thanks.