vsh: Define HAVE_STDARG_H before including readline

vsh: Define HAVE_STDARG_H before including readline

As it turned out my previous commits which switched from HAVE_ to
WITH_ and dropped stdarg.h detection were a bit too aggressive.
Because of reasons described in 9ea3424a178 we need to define
HAVE_STDARG_H before including readline otherwise macos build
fails. Honestly, I still don't fully understand the problem so I
am not going to bother you with "explanation".

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>