Fix dependencies

vendor/google.golang.org/protobuf/internal/strs/strings_pure.go

@@ -0,0 +1,27 @@
+// Copyright 2018 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 purego appengine
+
+package strs
+
+import pref "google.golang.org/protobuf/reflect/protoreflect"
+
+func UnsafeString(b []byte) string {
+	return string(b)
+}
+
+func UnsafeBytes(s string) []byte {
+	return []byte(s)
+}
+
+type Builder struct{}
+
+func (*Builder) AppendFullName(prefix pref.FullName, name pref.Name) pref.FullName {
+	return prefix.Append(name)
+}
+
+func (*Builder) MakeString(b []byte) string {
+	return string(b)
+}