Fix build error for missing class HashSet on some platforms
Fix build error for missing class HashSet on some platforms

evgenekov/protobuf-csharp-port,evgenekov/protobuf-csharp-port,plutoo/protobuf-csharp-port,plutoo/protobuf-csharp-port,evgenekov/protobuf-csharp-port,evgenekov/protobuf-csharp-port,plutoo/protobuf-csharp-port,happydpc/protobuf-csharp-port,happydpc/protobuf-csharp-port,happydpc/protobuf-csharp-port,happydpc/protobuf-csharp-port,plutoo/protobuf-csharp-port