unix: add SO_ORIGINAL_DST on linux

Change-Id: I243a05696e061f0f0e558518c8af596364aa3668
Reviewed-on: https://go-review.googlesource.com/c/sys/+/331609
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: tklauser

unix/linux/types.go

@@ -115,6 +115,7 @@ struct termios2 {
 #include <linux/netfilter/nf_tables_compat.h>
 #include <linux/netfilter/nfnetlink.h>
 #include <linux/netfilter.h>
+#include <linux/netfilter_ipv4.h>
 #include <linux/netlink.h>
 #include <linux/nexthop.h>
 #include <linux/nfc.h>
@@ -1767,6 +1768,8 @@ const (
 	NFPROTO_NUMPROTO = C.NFPROTO_NUMPROTO
 )
 
+const SO_ORIGINAL_DST = C.SO_ORIGINAL_DST
+
 // netfilter nfnetlink
 type Nfgenmsg C.struct_nfgenmsg