Add required feature for example async-std-echo.

Allows `cargo +nightly test` without any features.

Cargo.toml

@@ -79,6 +79,10 @@ async-std = { version = "1.0", features = ["attributes", "unstable"] }
 name = "autobahn-client"
 required-features = ["async-std-runtime"]
 
+[[example]]
+name = "async-std-echo"
+required-features = ["async-std-runtime"]
+
 [[example]]
 name = "client"
 required-features = ["async-std-runtime"]