Add rt-multi-thread feature for example of stdio

[cz-theng]

When run cargo run --example stdio in jsonrpc/stdio direcotry , got:

	error: The default runtime flavor is `multi_thread`, but the `rt-multi-thread` feature is disabled.
	 --> stdio/examples/stdio.rs:4:1
	  |
	4 | #[tokio::main]
	  | ^^^^^^^^^^^^^^
	  |
	  = note: this error originates in the attribute macro `tokio::main` (in Nightly builds, run with -Z macro-backtrace for more info)

	error: could not compile `jsonrpc-stdio-server` due to previous error

should open "rt-multi-thread" for #[tokio::main]