idiag: add a copy of linux/sock_diag.h

When we test idiag on 3.4 kernel, we always get ERANGE.
This is because libnl has its own copy for SK_MEMINFO_*,
which is actually newer than 3.4, where SK_MEMINFO_VARS
is larger than kernel's.

We add a copy from latest kernel, so on older kernel
libnl should still compile. Note, for kernel < 3.6
we don't have SK_MEMINFO_BACKLOG, we have to relax
the minlen.

'sock_diag.h' comes from v3.17 kernel sources
(bfe01a5ba2490f299e1d2d5508cbbbadd897bbe9), file
'include/uapi/linux/sock_diag.h'.

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