fix: add tracing debug only when tracing feature is enabled

[suryapandian]

Since the recent octocrab update we have started getting this error during build time

error[E0433]: failed to resolve: use of undeclared crate or module `tracing`
    --> /home/********/.cargo/registry/src/index.crates.io-6f17d22bba15001f/octocrab-0.30.0/src/lib.rs:1414:9
     |
1414 |         tracing::debug!("Token expires at: {:?}", expiration);
     |         ^^^^^^^ use of undeclared crate or module `tracing`

For more information about this error, try `rustc --explain E0433`.
error: could not compile `octocrab` (lib) due to previous error

Noticed that this PR introduces tracing::debug
When projects do not enable the tracing feature, we get this build error.

Please review this PR which will fix the compile time error

[XAMPPRocky]

Thank you for your PR!