crypto/x509: fix nacl build

Change-Id: Ie47c6460c1749aef3cf6d7c6ba44d43305d7ca7b
Reviewed-on: https://go-review.googlesource.com/2305Reviewed-by: Minux Ma <minux@golang.org>

src/crypto/x509/root_nacl.go

+// 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 x509
+
+// Possible certificate files; stop after finding one.
+var certFiles = []string{}