Windows: use Box::leak() instead of Box::into_raw()

This was detected by a new change in Nightly Rust that applied
`#[must_use]` to the return value of `Box::into_raw()`.

src/platform_impl/windows/event_loop.rs

@@ -2391,7 +2391,7 @@ unsafe extern "system" fn thread_event_target_callback(
  if userdata_removed {
  drop(userdata);
  } else {
- Box::into_raw(userdata);
+ Box::leak(userdata);
  }
  result
 }