[Patch, Fortran] add co_reduce to libcaf_single

[Janus Weil]

Hi Tobias,

> Looking through the stashed patches, I realized a pending (unsubmitted)
> patch, showing that both a test case for CO_REDUCE was missing and that
> libcaf_single didn't include co_reduce.
>
> This patch adds them.
>
> Build and regtested on x86-64-gnu-linux.
> OK for the trunk?

looks rather trivial to me. All the correctness checking for CO_REDUCE
is already there, right?

Cheers,
Janus