style(bindings): address new clippy lint

[jmayclin]

With the new rust version, we are seeing the following clippy error.

 error: in a `match` scrutinee, avoid complex blocks or closures with blocks; instead, move the block or closure higher and bind it with a `let`
Error:    --> s2n-tls-tokio/src/lib.rs:115:56
    |
115 |                   ready!(self.tls.with_io(ctx, |context| {
    |  ________________________________________________________^
116 | |                     let conn = context.get_mut().as_mut();
117 | |                     conn.poll_negotiate().map(|r| r.map(|_| ()))
118 | |                 }))
    | |_________________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#blocks_in_conditions
    = note: `-D clippy::blocks-in-conditions` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::blocks_in_conditions)]`

This PR fixes that clippy error.

Testing:

Existing CI should pass

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.