Unless he's become naturalized just in the past three years, Shatner is not a U.S. citizen. You can follow the links from here to the article you linked to and then to an interview in 2013, in which he talks about why he has chosen not to become a U.S. citizen.
Thanks for clearing this up.
In any way, of the four actors @Lance Charger named, only one can vote and has not publicly declared opposition to Donald Trump.