Make ui-fulldeps/update-references executable

When a ui-fulldeps comparison fails it suggests running
update-references.sh:

```
src/test/ui-fulldeps/update-references.sh 'rust/build/x86_64-apple-darwin/test/ui-fulldeps' 'resolve-error.rs'
```

This does not work as update-references.sh isn't executable. The other
update-references.sh in the ui directory is already executable so this looks
like an oversight.