tokio-xmpp: Fix build on insecure-tcp

insecure-tcp requires the io-util feature of tokio, so let’s enable it
there.

Fixes this error:
5   | use tokio::{io::BufStream, net::TcpStream};
    |             ^^^^^^^^^^^^^ no `BufStream` in `io`

tokio-xmpp/ChangeLog

@@ -33,6 +33,7 @@ XXXX-YY-ZZ RELEASER <admin@example.com>
         actual encryption and decryption to the kernel, which in turn can
         delegate it to a hardware implementations if available.  This depends
         on the `tls-rust-ktls` feature. (!458, !490)
+      - Building with insecure-tcp and no other feature got fixed.
 
 Version 4.0.0:
 2024-07-26 Maxime “pep” Buquet <pep@bouah.net>