rust-mode.el uses the 'cl macros, so it should actually require them

Without this change, rust-mode doesn't work if 'cl hasn't been required
by something else, apparently.  I'm not entirely sure what changed such
that I started seeing this problem instead of not, but maybe the emacs
world has been making progress towards not loading 'cl at runtime if
it's only needed at compile time.

(This change was previously submitted as e93a58d52 and accidentally reverted by ad8b437ad.)