vasprintf-posix, vasprintf-gnu: Fix module description. Reported by Vivien Kraus <vivien@planete-kraus.eu> in <https://lists.gnu.org/archive/html/bug-gnulib/2025-03/msg00009.html>. * modules/vasprintf-posix (Include): Change to <stdio.h>. * modules/vasprintf-gnu (Include): Likewise.