Fix cargo clippy build failure with Rust 1.82

[psychon]

CI reports:

error: irrefutable `if let` pattern
   --> x11rb-async/examples/xclock_utc_async.rs:243:20
    |
243 |                 if let Err(e) = drive.await {
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: this pattern will always match, so the `if let` is useless
    = help: consider replacing the `if let` with a `let`

Trying the suggested fix does not work with older rustc:

error[E0005]: refutable pattern in local binding
   --> x11rb-async/examples/xclock_utc_async.rs:243:21
    |
243 |                 let Err(e) = drive.await;
    |                     ^^^^^^ pattern `Ok(_)` not covered

So change this "if let" into a "match" that works everywhere.

[notgull]

Looks good to me!