scripts: generate_rust_analyzer: syn: treat `std` as a dependency

Fix the `generate_rust_analyzer.py` script to ensure that the
`rust-project.json` it produces includes `std` in the `deps` field for
the `syn` crate.

`syn` directly references items from `std`, so rust-analyzer should
treat it as a dependency to provide correct IDE support.

For example, `syn::Punctuated` contains fields of type `Vec<..>` and
`Option<..>`, both of which come from the standard library prelude.
With `std` listed in the `deps` field, rust-analyzer can infer the types
of these fields instead of showing `{unknown}`.

Verified the explicit uses of `std` using:

grep -rn 'std::' rust/syn/

Fixes: 737401751ace ("rust: syn: enable support in kbuild")
Signed-off-by: Jesung Yang <y.j3ms.n@gmail.com>
Reviewed-by: Tamir Duberstein <tamird@gmail.com>
Tested-by: Tamir Duberstein <tamird@gmail.com>
Link: https://patch.msgid.link/6dbdf6e1c1639ae381ca9ab7041f84728ffa2267.1764062688.git.y.j3ms.n@gmail.com
[ Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>