Lua: Silence warning

Change-Id: Iec6511b7fca648115f8a350b8de3962c325abf6c
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/lua/luauibindings.cpp

@@ -100,7 +100,7 @@ void qobject_index_set(QObject *obj, const char *key, sol::stack_object value)
 }
 
 template<class T>
-size_t qobject_index_size(QObject *obj)
+size_t qobject_index_size(QObject *)
 {
     return 0;
 }