vendor: add cfssl dependency

vendor/github.com/google/certificate-transparency/go/x509/root.go

@@ -0,0 +1,17 @@
+// Copyright 2012 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
+
+import "sync"
+
+var (
+	once        sync.Once
+	systemRoots *CertPool
+)
+
+func systemRootsPool() *CertPool {
+	once.Do(initSystemRoots)
+	return systemRoots
+}