CLEANUP: auth: make the code build again with DEBUG_AUTH

Reported by Cyril Bonté, minor issue caused by recent ACL rework.
diff --git a/src/auth.c b/src/auth.c
index f737fce..c9f7e04 100644
--- a/src/auth.c
+++ b/src/auth.c
@@ -137,7 +137,7 @@
 
 #ifdef DEBUG_AUTH
 	fprintf(stderr, "cfg: user=%s, pass=%s, group_mask=%u, flags=%X",
-		u->user, u->pass, u->group_mask, u->flags);
+		u->user, u->pass, u->u.group_mask, u->flags);
 #endif
 
 	/*