Yes, you can use human sunscreen on cats ears. I asked my vet when my little man was a kitten and he said it was fine to do so, and a lot safer than not using anything and I'm still doing so 10 years later with no problems.
I buy Soltan Once for kids and just apply it to the tips of his ears and let him out 15 mins later so I know he hasn't groomed it out before he settles down for a hard days sunbathing on the shed roof. You can use whichever one suits you but this one suits my cat
http://www.boots.com/webapp/wcs/stores/servlet/ProductDisplay?categoryParentId=4717&storeId=10052&productId=35647&callingViewName=&categoryId=49842&langId=-1&catalogId=11051