ospfd: justify ospf_default_originate_timer()

The function is implemented in ospf_lsa.c, move its "extern" declaration
to ospf_lsa.h for consistency.
diff --git a/ospfd/ospf_lsa.h b/ospfd/ospf_lsa.h
index bf3b083..f364840 100644
--- a/ospfd/ospf_lsa.h
+++ b/ospfd/ospf_lsa.h
@@ -275,6 +275,7 @@
 
 extern struct ospf_lsa *ospf_external_lsa_originate (struct ospf *, struct external_info *);
 extern int ospf_external_lsa_originate_timer (struct thread *);
+extern int ospf_default_originate_timer (struct thread *);
 extern struct ospf_lsa *ospf_lsa_lookup (struct ospf_area *, u_int32_t,
 				  struct in_addr, struct in_addr);
 extern struct ospf_lsa *ospf_lsa_lookup_by_id (struct ospf_area *,