configure: fix pam test warning

configure: fix pam test warning

The pam test generates a warning on Fedora 29 with -O3 compilation
because the headers declare that the pam_conversation pointer to
pam_start must be non-NULL.  Change it to use the same 0 initialised
structure as we actually use in qauthz.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Acked-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190404091725.20595-1-dgilbert@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>