listing.c: handle multiple error messages on a single line
listing.c: handle multiple error messages on a single line

We may produce an arbitrary number of error messages on a single line;
include all of them in the list file.

Signed-off-by: H. Peter Anvin (Intel) <8a453bad9912ffe59bc0f0b8abe03df9be19379e@zytor.com>

techkey/nasm,techkey/nasm,techkey/nasm,techkey/nasm,techkey/nasm