[tlbuild] Tweak for harfbuzz

[Ken Brown]

On 1/10/2018 4:39 PM, Karl Berry wrote:
>      looked into it more deeply and found that this is due to an error in the
>      harfbuzz sources.
> 
> Thanks. I hoped this would happen :).
> I'll apply the patch in a little while for the interim.
> 
> I hope this means that all the other painful changes we committed
> yesterday (new poppler, new luatex, C++11 for both) do build on Cygwin ...k

Yes, everything else was fine.

Ken