<stdio.h>: Acknowledge that getdelim/getline are in POSIX

These comments were written in 2003 (added in 2c008571), predating
the addition of getdelim(3)/getline(3) in POSIX.1-2008.

Reviewed-by: Sam James <sam@gentoo.org>
Reviewed-by: Florian Weimer <fweimer@redhat.com>