Expansion should explicitly include enum

In preparation for [removing the `std::cmp::Ordering` reexport](https://github.com/rust-lang/rust/issues/19253), this needs to be done to prevent errors like:

```
note: in expansion of #[deriving]
note: expansion site
error: unresolved name `std::cmp::Equal`
#[deriving(Clone, PartialEq, PartialOrd, Eq, Ord, Show)]
                                             ^~~
```