meson: actually #define HAVE_ASPRINTF

I believe this should now correctly enable the code iff it's needed.

This commits and the parent will probably be no-op in practice.
I believe distros commonly do use fortification by default,
and I'm not aware of any platform with kresd and without asprintf().

I considered using `conf_data.set()` in meson, but that would require
us modifying contrib/ccan/asprintf/asprintf.h to include kresconfig.h,
and that seemed weird.