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)]
                                              ^~~
```

src/libsyntax/ext/deriving/cmp/totalord.rs

@@ -64,6 +64,7 @@ pub fn cs_cmp(cx: &mut ExtCtxt, span: Span,
     let equals_path = cx.path_global(span,
                                      vec!(cx.ident_of("std"),
                                           cx.ident_of("cmp"),
+                                          cx.ident_of("Ordering"),
                                           cx.ident_of("Equal")));
 
     let cmp_path = vec![