Fix loading of cider-stacktrace.el

Fixes an issue where cider-stacktrace.el was not loading correctly, and required
a manual re-evaluation before it would show stacktraces.
No, I would say not. I didn't introduce that line -- I just moved the function as-is from `cider-repl.el` to `cider-util.el`. I think that is cruft from a much earlier era.