genpolicy: Adjust to build with rust 1.80.0

```
error: field `image` is never read
  --> src/registry.rs:35:9
   |
34 | pub struct Container {
   |            --------- field in this struct
35 |     pub image: String,
   |         ^^^^^
   |
   = note: `Container` has derived impls for the traits `Debug` and `Clone`, but these are intentionally ignored during dead code analysis
   = note: `-D dead-code` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(dead_code)]`

error: field `use_cache` is never read
   --> src/utils.rs:106:9
    |
105 | pub struct Config {
    |            ------ field in this struct
106 |     pub use_cache: bool,
    |         ^^^^^^^^^
    |
    = note: `Config` has derived impls for the traits `Debug` and `Clone`, but these are intentionally ignored during dead code analysis

error: could not compile `genpolicy` (bin "genpolicy") due to 2 previous errors
```

Signed-off-by: Fabiano Fidêncio <fabiano@fidencio.org>
Signed-off-by: stevenhorsman <steven@uk.ibm.com>

src/tools/genpolicy/src/registry.rs

@@ -32,6 +32,7 @@ use tokio::io::AsyncWriteExt;
 /// Container image properties obtained from an OCI repository.
 #[derive(Clone, Debug, Default)]
 pub struct Container {
+    #[allow(dead_code)]
     pub image: String,
     pub config_layer: DockerConfigLayer,
     pub image_layers: Vec<ImageLayer>,

src/tools/genpolicy/src/utils.rs

@@ -103,6 +103,7 @@ struct CommandLineOptions {
 /// Application configuration, derived from on command line parameters.
 #[derive(Clone, Debug)]
 pub struct Config {
+    #[allow(dead_code)]
     pub use_cache: bool,
     pub insecure_registries: Vec<String>,
     pub runtime_class_names: Vec<String>,