pv: Fix unfulfilled_lint_expectations warning

The test cases uses the ml_kem functions, therefore disable the
directive for tests.

$ cargo test
...
 --> pv/src/openssl_extensions/ml_kem.rs:5:11
  |
5 | #![expect(unused)]
  |           ^^^^^^
  |
  = note: `#[warn(unfulfilled_lint_expectations)]` on by default

Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Marc Hartmayer <marc@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

rust/pv/src/openssl_extensions/ml_kem.rs

@@ -2,7 +2,7 @@
 //
 // Copyright IBM Corp.
 
-#![expect(unused)]
+#![cfg_attr(not(test), expect(unused))]
 
 use std::ffi::CStr;
 use std::ptr::NonNull;