unix: fix scandir filter prototype again
unix: fix scandir filter prototype again

The only platforms where the dirent argument is non-const are OS X, OpenBSD
and older versions of FreeBSD (but not FreeBSD 9). Accommodate the first two.

urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso