add vendoring with go dep

vendor/golang.org/x/net/ipv6/sys_asmreq.go

@@ -0,0 +1,24 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris windows
+
+package ipv6
+
+import (
+	"net"
+	"unsafe"
+
+	"golang.org/x/net/internal/socket"
+)
+
+func (so *sockOpt) setIPMreq(c *socket.Conn, ifi *net.Interface, grp net.IP) error {
+	var mreq ipv6Mreq
+	copy(mreq.Multiaddr[:], grp)
+	if ifi != nil {
+		mreq.setIfindex(ifi.Index)
+	}
+	b := (*[sizeofIPv6Mreq]byte)(unsafe.Pointer(&mreq))[:sizeofIPv6Mreq]
+	return so.Set(c, b)
+}