stats: use llist_add_tail() in osmo_stats_reporter_alloc()

This allows printing reporters in the exact order as they were configired.

Change-Id: I904cd0ed53510dbe26c15cd287ba2707ca04cd6e
Related: SYS#5713
diff --git a/src/stats.c b/src/stats.c
index 28a3ab3..0967305 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -223,7 +223,7 @@
 		srep->name = talloc_strdup(srep, name);
 	srep->fd = -1;
 
-	llist_add(&srep->list, &osmo_stats_reporter_list);
+	llist_add_tail(&srep->list, &osmo_stats_reporter_list);
 
 	return srep;
 }