prevent a notice from being shown if user is not a member of duo-users.

this is to reduce confusion over a limited scope rollout of duo
diff --git a/pam_duo/pam_duo.c b/pam_duo/pam_duo.c
index 75fc962..b6dbe0d 100644
--- a/pam_duo/pam_duo.c
+++ b/pam_duo/pam_duo.c
@@ -220,7 +220,8 @@
         return (PAM_SERVICE_ERR);
     } else if (matched == 0) {
         if (cfg.group_access_fail) {
-          duo_syslog(LOG_INFO, "User %s not a member of an authorized UNIX group for Duo 2FA auth.", user);
+          /* suppress info notice for transparent fallthrough to another factor */
+          /* duo_syslog(LOG_INFO, "User %s not a member of an authorized UNIX group for Duo 2FA auth.", user); */
           close_config(&cfg);
           return (PAM_SERVICE_ERR);
         } else {