I know pumpkin leaves are quite nutritious (being high in vitamin A, calcium and other trace elements) and are okay for humans to eat but I don't know about piggies. The fact they are high in calcium and vitamin A probably makes them less suitable as a food for pigs even if they are otherwise safe. It is possible for plants in the cucumber family to have toxic levels of nitrate in the leaves if they are over fertilised and they are rather bristly/prickly (but pigs eat other bristly leaves without issue so that might not be such a problem). As ever, if in doubt don't feed it to them.