I think you should tell your roommate. A lot of people wouldn’t care, but there are still a lot of people out there who would be uncomfortable rooming with a guy. Personally, I’d want to know, but only so that I could use his preferred pronouns and name around people who also know he’s trans. Maybe you’ll get lucky and she’ll also be LGBTQA, and then she wouldn’t dream of judging you for it.
Perhaps you can do what the others have said and take a few days to get to know her before letting her know, so that she knows you’re a nice person and would be less likely to judge you immediately, if she is conservative?
I think you should tell her for your own happiness. It’s already going to be very uncomfortable for you to live on a hall of all girls, as if you’re one of them, right? It’d be nice to have at least one person who knows what’s up. It really is a shame you couldn’t get a single or gender neutral housing.