update to latest adns, synced with Go's net

vendor/github.com/mjl-/adns/hook_windows.go

@@ -0,0 +1,18 @@
+// Copyright 2015 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.
+
+package adns
+
+import (
+	"golang.org/x/sys/windows"
+)
+
+func windowsGetSystemDirectory() string {
+	p, _ := windows.GetSystemDirectory()
+	return p
+}
+
+var (
+	hostsFilePath = windowsGetSystemDirectory() + "/Drivers/etc/hosts"
+)