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: f301cb978c06 ("rust: devres: implement Devres::access()")
Signed-off-by: Miguel Ojeda <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Danilo Krummrich <[e-mail]>

rust/kernel/devres.rs

···
     /// # 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 {