Add (require 'cl), required to use loop.

Does not compile cleanly, nor work, without it:
```
In helm-auto-complete-candidates:
ac-helm.el:94:9:Warning: reference to free variable `for'
ac-helm.el:94:13:Warning: reference to free variable `x'
ac-helm.el:94:15:Warning: reference to free variable `in'
ac-helm.el:94:45:Warning: reference to free variable `collect'

In end of data:
ac-helm.el:118:1:Warning: the function `loop' is not known to be defined.
```