Ignore mutex_atomic clippy lint

> This lint cannot detect if the mutex is actually used for waiting
before a critical section.

src/unlock_notify.rs

@@ -17,6 +17,7 @@ struct UnlockNotification {
 }
 
 #[cfg(feature = "unlock_notify")]
+#[allow(clippy::mutex_atomic)]
 impl UnlockNotification {
     fn new() -> UnlockNotification {
         UnlockNotification {