tools headers UAPI: Sync linux/in.h copy with the kernel sources

To pick the changes in:

  db243b79 ("net/ipv4/ipv6: Replace one-element arraya with flexible-array members")
  2d3e5caf

 ("net/ipv4: Replace one-element array with flexible-array member")

That don't result in any change in tooling, the structs changed remains
with the same layout.

This addresses this build warning:

  Warning: Kernel ABI header at 'tools/include/uapi/linux/in.h' differs from latest version at 'include/uapi/linux/in.h'
  diff -u tools/include/uapi/linux/in.h include/uapi/linux/in.h

Cc: David S. Miller <davem@davemloft.net>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>