Don't inject stuff into the std namespace in Qhull.

Clang 3.5.0 didn't like this particular line of code, but it was fixed
in a straightforward way just by including <iterator>.

For reference, the error was:

> error: reference to 'random_access_iterator_tag' is ambiguous
> typedef std::random_access_iterator_tag  iterator_category;
>
> note: candidate found by name lookup is 'std::__1::random_access_iterator_tag'
> struct _LIBCPP_TYPE_VIS_ONLY random_access_iterator_tag : public bidirectional_iterator_tag {};
>
> note: candidate found by name lookup is 'std::random_access_iterator_tag'
> namespace std { struct bidirectional_iterator_tag; struct random_access_iterator_tag; }