It can be hard to find privacy when you are sharing your space with other people. I would think that maybe you can have a conversation with your roommate about how you live together and give each other space and privacy. This doesn't have to mean talking about masturbation! More sitting down together and agreeing what things you'd like in place to make sure everyone is comfortable. This includes how clean you want to keep your room, when and if you can bring friends around and how you respect alone time. Maybe you can agree things like saying how long you'll be out for, and texting if you're coming back sooner so that you can feel comfortable that you have some time. What do you think?
Also, whilst I'm not saying you need to talk to your roommate explicitly about masturbation, it is really nothing to be ashamed of! There is nothing bad or wrong about masturbating, most people do or have at some point in their lives so I'm certain you're not the only one at your college hostel having to figure this out. You might like this: Is Masturbation Okay? (Yep.)
. Does that help?