MINOR: lua: add the struct session in the lua channel struct

This is used later to modify some flags in the session.
diff --git a/include/types/hlua.h b/include/types/hlua.h
index 58402f4..b1d99f4 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -121,6 +121,7 @@
  */
 struct hlua_channel {
 	struct channel *chn;
+	struct session *s;
 };
 
 #else /* USE_LUA */