Ignore unexpected_cfgs lint for generated code

```
error: unexpected `cfg` condition name: `rustfmt`
 --> src/gen/ast_struct.rs:6:13
  |
6 | #![cfg_attr(rustfmt, rustfmt::skip)]
  |             ^^^^^^^
  |
  = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(rustfmt)");` to the top of the `build.rs`
  = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

error: unexpected `cfg` condition name: `rustfmt`
 --> src/gen/ast_enum.rs:6:13
  |
6 | #![cfg_attr(rustfmt, rustfmt::skip)]
  |             ^^^^^^^
  |
  = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(rustfmt)");` to the top of the `build.rs`
  = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration

error: unexpected `cfg` condition name: `rustfmt`
 --> src/gen/convert.rs:6:13
  |
6 | #![cfg_attr(rustfmt, rustfmt::skip)]
  |             ^^^^^^^
  |
  = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(rustfmt)");` to the top of the `build.rs`
  = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
```

Due to rust-lang/rust#124735, we cannot
disable this lint at module-level.

src/lib.rs

@@ -130,6 +130,7 @@ The data structures of syn-serde 0.3 is compatible with the data structures of
 )]
 // docs.rs only (cfg is enabled by docs.rs, not build script)
 #![cfg_attr(docsrs, feature(doc_cfg))]
+#![allow(unexpected_cfgs)] // https://github.com/rust-lang/rust/issues/124735
 
 #[macro_use]
 mod macros;

tools/codegen/src/file.rs

@@ -31,7 +31,9 @@ pub(crate) fn workspace_root() -> PathBuf {
 #[track_caller]
 pub(crate) fn header(function_name: &str) -> String {
     // rust-analyzer does not respect outer attribute (#[rustfmt::skip]) on
-    // a module without a body. So use inner attribute under cfg(rustfmt).
+    // a module without a body and unstable ignore option in .rustfmt.toml.
+    // https://github.com/rust-lang/rust-analyzer/issues/10826
+    // So use inner attribute under cfg(rustfmt).
     format!(
         "// SPDX-License-Identifier: Apache-2.0 OR MIT
 // This file is @generated by {bin_name}