rust: iommu: fix srctree link warning

The Rust kernel code should be kept `rustdoc`-clean [1].

Our custom `srctree` link checker in the `rustdoc` target reports:

    warning: srctree/ link to include/io-pgtable.h does not exist

Thus fix it.

Link: https://rust-for-linux.com/contributing#submit-checklist-addendum [1]
Fixes: 2e2f6b0 ("rust: iommu: add io_pgtable abstraction")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

Author: ojeda

rust/kernel/iommu/pgtable.rs

@@ -2,7 +2,7 @@
 
 //! IOMMU page table management.
 //!
-//! C header: [`include/io-pgtable.h`](srctree/include/io-pgtable.h)
+//! C header: [`include/linux/io-pgtable.h`](srctree/include/linux/io-pgtable.h)
 
 use core::{
     marker::PhantomData,