codegen: allow(unreachable_patterns)

```
error: unreachable pattern
    --> relay-clients/client-polkadot/src/codegen_runtime.rs:5751:22
     |
5751 |                 Void(runtime_types::sp_core::Void),
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ matches no values because `codegen_runtime::api::runtime_types::sp_core::Void` is uninhabited
     |
     = note: to learn more about uninhabited types, see https://doc.rust-lang.org/nomicon/exotic-sizes.html#empty-types
     = note: `-D unreachable-patterns` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(unreachable_patterns)]`
```

codegen/src/api/mod.rs

@@ -81,7 +81,7 @@ impl RuntimeGenerator {
 
         Ok(quote! {
             #( #item_mod_attrs )*
-            #[allow(dead_code, unused_imports, non_camel_case_types)]
+            #[allow(dead_code, unused_imports, non_camel_case_types, unreachable_patterns)]
             #[allow(clippy::all)]
             #[allow(rustdoc::broken_intra_doc_links)]
             pub mod #mod_ident {
@@ -233,7 +233,7 @@ impl RuntimeGenerator {
 
         Ok(quote! {
             #( #item_mod_attrs )*
-            #[allow(dead_code, unused_imports, non_camel_case_types)]
+            #[allow(dead_code, unused_imports, non_camel_case_types, unreachable_patterns)]
             #[allow(clippy::all)]
             #[allow(rustdoc::broken_intra_doc_links)]
             pub mod #mod_ident {