rust: devres: fix doctest build under !CONFIG_PCI

The doctest requires `CONFIG_PCI`:

    error[E0432]: unresolved import `kernel::pci`
        --> rust/doctests_kernel_generated.rs:2689:44
         |
    2689 | use kernel::{device::Core, devres::Devres, pci};
         |                                            ^^^ no `pci` in the root
         |
    note: found an item that was configured out
        --> rust/kernel/lib.rs:96:9
    note: the item is gated here
        --> rust/kernel/lib.rs:95:1

Thus conditionally compile it (which still checks the syntax).

Fixes: f301cb9 ("rust: devres: implement Devres::access()")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20250511182533.1016163-1-ojeda@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: ojeda

rust/kernel/devres.rs

@@ -195,6 +195,7 @@ impl<T> Devres<T> {
     /// # Example
     ///
     /// ```no_run
+    /// # #![cfg(CONFIG_PCI)]
     /// # use kernel::{device::Core, devres::Devres, pci};
     ///
     /// fn from_core(dev: &pci::Device<Core>, devres: Devres<pci::Bar<0x4>>) -> Result {