fix(macos): suppress unused_variables warning reported only in release build

[rhysd]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

This PR fixes the warning when building this crate in release mode as follows:

$ cargo build --release
   Compiling wry v0.23.1 (/Users/rhysd/Develop/github.com/tauri-apps/wry)
warning: unused variable: `description`
   --> src/webview/wkwebview/download.rs:105:9
    |
105 |     let description = NSString(description).to_str().to_string();
    |         ^^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_description`
    |
    = note: `#[warn(unused_variables)]` on by default

warning: `wry` (lib) generated 1 warning
    Finished release [optimized] target(s) in 2.31s

The description variable is used only when debug_assertions is enabled.

[wusyong]

How about we move these to debug_assertions below? And rename error to _error.

#[cfg(debug_assertions)]
{
    let description: id = msg_send![_error, localizedDescription];
    let description = NSString(description).to_str().to_string();
    eprintln!("Download failed with error: {}", description);
}

[rhysd]

Looks better since allocation can be avoided. I'll have a try.

[rhysd]

@wusyong I revised my patch and force-pushed as e73c4e6