Expansion should explicitly include enum

In preparation for removing the std::cmp::Ordering reexport, 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)]
                                              ^~~
```
Merge pull request #19812 from frewsxcv/expansion-include-enum

Expansion should explicitly include enum

Reviewed-by: alexcrichton