crypto/tls: make compatible with go/build

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5484073

root_stub.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build plan9
+
 package tls
 
 func initDefaultRoots() {

root_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build freebsd linux openbsd netbsd
+
 package tls
 
 import (