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

@@ -2441,7 +2441,7 @@ unsafe extern "system" fn thread_event_target_callback(
     if userdata_removed {
         drop(userdata);
     } else {
-        Box::into_raw(userdata);
+        Box::leak(userdata);
     }
     result
 }