PR port-amiga/56188

Include a prototype for __nbcompat_heapsort. The tools for amiga now
build on Darwin where one Amiga specific tool uses heapsort.

Despite being a one-liner, ./build.sh -m amiga tools was tested on
NetBSD, Darwin, FreeBSD & Linux with the change.

Discussed various options with jdc & thorpej. Perhaps in the future
we could look at using qsort instead of heapsort as qsort is
standardised and more likely to be available. We could then remove
heapsort from the compatibility library.

(cjep)