bgpd: cleanup privs on termination

When doing valgrind testing, the privledges from zprivs_init() need
to be cleaned up on exit.
diff --git a/bgpd/bgp_main.c b/bgpd/bgp_main.c
index 778067a..f835fe7 100644
--- a/bgpd/bgp_main.c
+++ b/bgpd/bgp_main.c
@@ -199,6 +199,7 @@
   if (! retain_mode)
     bgp_terminate ();
 
+  zprivs_terminate (&bgpd_privs);
   bgp_exit (0);
 }