vtysh: make warnings about node installs a non-default compile-time option

* lib/command.c: (install_element) Suppress duplicate command install warnings
  for vtysh
diff --git a/lib/command.c b/lib/command.c
index 242c98b..bb94c28 100644
--- a/lib/command.c
+++ b/lib/command.c
@@ -638,9 +638,11 @@
   
   if (hash_lookup (cnode->cmd_hash, cmd) != NULL)
     {
+#ifdef DEV_BUILD
       fprintf (stderr, 
                "Multiple command installs to node %d of command:\n%s\n",
                ntype, cmd->string);
+#endif
       return;
     }