Fix vec-add

From https://github.com/mozilla/rust/blob/master/src/test/compile-fail/vec-add.rs

```
// FIXME: + should allow immutable or mutable vectors on the right
// hand side in all cases. We are getting compiler errors about this
// now, so I'm xfailing the test for now. -eholk

```
