rust: pin-init: internal: skip rustfmt formatting of kernel-only module

rust: pin-init: internal: skip rustfmt formatting of kernel-only module

The `quote` module only is available in the kernel and thus running
`cargo fmt` or `rustfmt internal/src/lib.rs` in the user-space
repository [1] results in:

    error: couldn't read `~/pin-init/internal/src/../../../macros/quote.rs`: No such file or directory (os error 2)
      --> ~/pin-init/internal/src/lib.rs:25:1
       |
    25 | mod quote;
       | ^^^^^^^^^^

    Error writing files: failed to resolve mod `quote`: ~/pin-init/internal/src/../../../macros/quote.rs does not exist

Thus mark it with `rustfmt::skip` when compiling without kernel support.

Link: https://github.com/Rust-for-Linux/pin-init
Link: https://github.com/Rust-for-Linux/pin-init/pull/33/commits/a6caf1945e51da38761aab4dffa56e63e2baa218
Link: https://lore.kernel.org/all/20250414195928.129040-2-benno.lossin@proton.me
Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>