Publish the starttls helper function

Without it, it seems impossible to start TLS on an XMPPStream object,
preventing any use of that object on its own (not wrapped into
AsyncClient or somesuch).

tokio-xmpp/src/lib.rs

@@ -17,3 +17,4 @@ mod component;
 pub use crate::component::Component;
 mod error;
 pub use crate::error::{AuthError, ConnecterError, Error, ParseError, ParserError, ProtocolError};
+pub use starttls::starttls;