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 <[email protected]>
Link: rust-lang/rust#132748 [1]
Fixes: 78418f3 ("rust/kernel: Add faux device bindings")
Signed-off-by: Miguel Ojeda <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: ojeda

rust/kernel/faux.rs

@@ -4,7 +4,7 @@
 //!
 //! This module provides bindings for working with faux devices in kernel modules.
 //!
-//! C header: [`include/linux/device/faux.h`]
+//! C header: [`include/linux/device/faux.h`](srctree/include/linux/device/faux.h)
 
 use crate::{bindings, device, error::code::*, prelude::*};
 use core::ptr::{addr_of_mut, null, null_mut, NonNull};