unix: add type NdUseroptmsg on Linux

Change-Id: I06997987f2d54a1fbca098878a3bac473ec1bfc0
Reviewed-on: https://go-review.googlesource.com/c/163379
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/linux/types.go

@@ -662,6 +662,7 @@ const (
 	SizeofIfAddrmsg      = C.sizeof_struct_ifaddrmsg
 	SizeofRtMsg          = C.sizeof_struct_rtmsg
 	SizeofRtNexthop      = C.sizeof_struct_rtnexthop
+	SizeofNdUseroptmsg   = C.sizeof_struct_nduseroptmsg
 )
 
 type NlMsghdr C.struct_nlmsghdr
@@ -682,6 +683,8 @@ type RtMsg C.struct_rtmsg
 
 type RtNexthop C.struct_rtnexthop
 
+type NdUseroptmsg C.struct_nduseroptmsg
+
 // Linux socket filter
 
 const (