Fix gating of tokio feature

[kpcyrd]

Debian tooling noticed the crate doesn't build with:

cargo check --no-default-features -F tokio

because src/irc.rs is gated like this:

#[cfg(feature = "tokio")]
pub mod irc;

but contains imports like this:

use bytes::BytesMut;
use tokio_util::codec::{Decoder, Encoder};

With this patch tokio becomes a feature that enables the relevant other dependencies. :)