MINOR: lua: add class listener

This class provides the access to the listener struct, it allows
some manipulations and retrieve informations.
diff --git a/include/types/hlua.h b/include/types/hlua.h
index afe870a..43e9f1a 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -21,6 +21,7 @@
 #define CLASS_APPLET_HTTP  "AppletHTTP"
 #define CLASS_PROXY        "Proxy"
 #define CLASS_SERVER       "Server"
+#define CLASS_LISTENER     "Listener"
 
 struct stream;