[PATCH] add gthr-gnat.c to LIB2ADDEH for darwin

[Arnaud Charlet]

> To support Ada on darwin, I need the following patch which adds
> gthr-gnat.c to LIB2ADDEH in config/t-darwin.
> OK?

Please don't commit this change: I am working on cleaning up the
gthr-gnat issue which is not satisfactory, and will likely end up removing
all references to gthr-gnat and co.

Hopefully this clean up will be terminated soon, so I'd suggest you
use this work around locally in the mean time, thanks.

Arno