rust: proc-macro2: rebuild if the version text changes

rust: proc-macro2: rebuild if the version text changes

The Rust compiler cannot use dependencies built by other versions, e.g.:

    error[E0514]: found crate `proc_macro2` compiled by an incompatible version of rustc
     --> rust/quote/ext.rs:5:5
      |
    5 | use proc_macro2::{TokenStream, TokenTree};
      |     ^^^^^^^^^^^
      |
      = note: the following crate versions were found:
              crate `proc_macro2` compiled by rustc 1.92.0 (ded5c06cf 2025-12-08): ./rust/libproc_macro2.rlib
      = help: please recompile that crate using this compiler (rustc 1.93.0 (254b59607 2026-01-19)) (consider running `cargo clean` first)

Thus trigger a rebuild if the version text changes like we do in other
top-level cases (e.g. see commit aeb0e24abbeb ("kbuild: rust: replace
proc macros dependency on `core.o` with the version text")).

The build errors for now are hard to trigger, since we do not yet use
the new crates we just introduced (the use cases are coming in the next
merge window), but they can still be seen if e.g. one manually removes
one of the targets, so fix it already.

Fixes: 158a3b72118a ("rust: proc-macro2: enable support in kbuild")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260122054135.138445-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>