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>

configure

@@ -2940,9 +2940,9 @@ if test "$auth_pam" != "no"; then
 int main(void) {
    const char *service_name = "qemu";
    const char *user = "frank";
-   const struct pam_conv *pam_conv = NULL;
+   const struct pam_conv pam_conv = { 0 };
    pam_handle_t *pamh = NULL;
-   pam_start(service_name, user, pam_conv, &pamh);
+   pam_start(service_name, user, &pam_conv, &pamh);
    return 0;
 }
 EOF