perror: adjust array size

perror: adjust array size

If we ever adjust strerror-override.h to have a larger size for
STACKBUF_LEN, then perror should also pick up the adjustment.

* modules/perror (Depends-on): Add strerror-override.
* lib/perror.c (perror): Use it to avoid magic number.

Signed-off-by: Eric Blake <eblake@redhat.com>