vfio-user: Fix clippy warning mismatched_lifetime_syntaxes

Fix clippy warning as Rust toolchain 1.89.0-beta.3 (b5e10d8c0
2025-07-02) suggested.

```console
Error:    --> vfio-user/examples/gpio/pci.rs:558:21
    |
558 |     pub fn get_bars(&self) -> PciBarIter {
    |                     ^^^^^     ---------- the lifetime gets resolved as `'_`
    |                     |
    |                     this lifetime flows to the output
    |
    = note: `-D mismatched-lifetime-syntaxes` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(mismatched_lifetime_syntaxes)]`
help: one option is to remove the lifetime for references and use the anonymous lifetime for paths
    |
558 |     pub fn get_bars(&self) -> PciBarIter<'_> {
    |                                         ++++
```

Signed-off-by: Ruoqing He <[email protected]>

Author: RuoqingHe

vfio-user/examples/gpio/pci.rs

@@ -555,7 +555,7 @@ impl PciConfiguration {
 
     /// Returns an iterator of the currently configured base address registers.
     #[allow(dead_code)] // TODO(dverkamp): remove this once used
-    pub fn get_bars(&self) -> PciBarIter {
+    pub fn get_bars(&self) -> PciBarIter<'_> {
         PciBarIter {
             config: self,
             bar_num: 0,