Git is probably already tracking the file.
From the gitignore docs:
To stop tracking a file that is currently tracked, use git rm –cached.
Use this, replacing [project] and [username] with your info:
git rm --cached [project].xcodeproj/project.xcworkspace/xcuserdata/[username].xcuserdatad/UserInterfaceState.xcuserstate
git commit -m "Removed file that shouldn't be tracked"
Alternatively you can use the -a option to git commit that will add all files that have been modified or deleted.
Once you’ve removed the file from git, it will respect your .gitignore.