chore: fixed std wasm build of xcm (#2535)

# Description

just ensures

```sh
cargo check --package staging-xcm  --no-default-features  --target wasm32-unknown-unknown --features=std
```

work (for CosmWasm)

# Checklist

- [x] My PR includes a detailed description as outlined in the
"Description" section above
- [ ] My PR follows the [labeling requirements](CONTRIBUTING.md#Process)
of this project (at minimum one label for `T`
  required)
- [x] I have made corresponding changes to the documentation (if
applicable)
- [ ] I have added tests that prove my fix is effective or that my
feature works (if applicable)

---------

Co-authored-by: Bastian Köcher <[email protected]>