Fix Rust 2024 unsafe block warnings in PAM callback

Wrap raw pointer operations in explicit unsafe blocks inside
the unsafe extern "C" conv callback, as required by Rust 2024
edition. Remove unused mut binding.

src/lockscreen.rs

@@ -47,7 +47,7 @@ pub fn create_lockscreen_window(
         }
     };
 
-    let mut fp_listener = FingerprintListener::new();
+    let fp_listener = FingerprintListener::new();
     let fp_available = config.fingerprint_enabled
         && fp_listener.is_available(&user.username);