chore(tls): lints.rust expects cfg(fuzzing)

this fixes an error observed in the nightly toolchain ci action.

https://github.com/linkerd/linkerd2-proxy/actions/runs/10728368500/job/29752764638?pr=3177#step:7:165

```
Error: error: unexpected `cfg` condition name: `fuzzing`
   --> linkerd/tls/src/server.rs:342:7
    |
342 | #[cfg(fuzzing)]
    |       ^^^^^^^
    |
    = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, and `windows`
    = help: consider using a Cargo feature instead
    = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
             [lints.rust]
             unexpected_cfgs = { level = "warn", check-cfg = ['cfg(fuzzing)'] }
    = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(fuzzing)");` to the top of the `build.rs`
```

Signed-off-by: katelyn martin <[email protected]>

linkerd/tls/Cargo.toml

@@ -6,6 +6,9 @@ license = "Apache-2.0"
 edition = "2021"
 publish = false
 
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(fuzzing)'] }
+
 [dependencies]
 async-trait = "0.1"
 bytes = "1"