rust: faux: fix C header link

rust: faux: fix C header link

Starting with Rust 1.91.0 (expected 2025-10-30), `rustdoc` has improved
some false negatives around intra-doc links [1], and it found a broken
intra-doc link we currently have:

    error: unresolved link to `include/linux/device/faux.h`
     --> rust/kernel/faux.rs:7:17
      |
    7 | //! C header: [`include/linux/device/faux.h`]
      |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `include/linux/device/faux.h` in scope
      |
      = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
      = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
      = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`

Our `srctree/` C header links are not intra-doc links, thus they need
the link destination.

Thus fix it.

Cc: stable <stable@kernel.org>
Link: https://github.com/rust-lang/rust/pull/132748
Fixes: 78418f300d39 ("rust/kernel: Add faux device bindings")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250804171311.1186538-1-ojeda@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>