Ignore dead_code warning for tuple struct

This lint does not take into account destructors.

```
error: field `0` is never read
    --> src\iocp\mod.rs:1155:13
     |
1155 |     Waiting(WaitHandle),
     |     ------- ^^^^^^^^^^
     |     |
     |     field in this variant
     |
     = note: `-D dead-code` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(dead_code)]`
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
     |
1155 |     Waiting(()),
     |             ~~
```

Author: taiki-e

src/iocp/mod.rs

@@ -1152,7 +1152,7 @@ enum WaitableStatus {
     Idle,
 
     /// We are waiting on this handle to become signaled.
-    Waiting(WaitHandle),
+    Waiting(#[allow(dead_code)] WaitHandle),
 
     /// This handle has been cancelled.
     Cancelled,