adding UnimplementedGreeterServer to server struct (#1973)
adding UnimplementedGreeterServer to server struct (#1973)

The compiler produces an error without this. The error is:

```
cannot use &server literal (type *server) as type helloworld.GreeterServer in argument to helloworld.RegisterGreeterServer:
	*server does not implement helloworld.GreeterServer (missing helloworld.mustEmbedUnimplementedGreeterServer method)
```
gengo/grpc-gateway,grpc-ecosystem/grpc-gateway,gengo/grpc-gateway