genpolicy: Fix clippy unstable_name_collisions

Manually fix `unstable_name_collisions` clippy warning reported by rust
1.85.1.

```console
error: a method with this name may be added to the standard library in the future
   --> src/registry.rs:646:10
    |
646 |     file.unlock()?;
    |          ^^^^^^
    |
    = warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
    = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
    = help: call with fully qualified syntax `fs2::FileExt::unlock(...)` to keep using the current method
    = note: `-D unstable-name-collisions` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unstable_name_collisions)]`
```

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

src/tools/genpolicy/src/registry.rs

@@ -602,7 +602,7 @@ pub fn add_verity_and_users_to_store(
     let mut writer = BufWriter::new(&file);
     writeln!(writer, "{}", serialized)?;
     writer.flush()?;
-    file.unlock()?;
+    fs2::FileExt::unlock(&file)?;
     Ok(())
 }