rust: work around `bindgen` 0.69.0 issue

`bindgen` 0.69.0 contains a bug: `--version` does not work without
providing a header [1]:

    error: the following required arguments were not provided:
      <HEADER>

    Usage: bindgen <FLAGS> <OPTIONS> <HEADER> -- <CLANG_ARGS>...

Thus, in preparation for supporting several `bindgen` versions, work
around the issue by passing a dummy argument.

Include a comment so that we can remove the workaround in the future.

Link: https://github.com/rust-lang/rust-bindgen/pull/2678

 [1]
Reviewed-by: Finn Behrens <me@kloenk.dev>
Tested-by: Benno Lossin <benno.lossin@proton.me>
Tested-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20240709160615.998336-9-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>