Add missing "use" of wcf\data\user\UserProfile

Currently users can't buy groups but the sum is being deducted from their account.

@see https://tims.bastelstu.be/forum/index.php/Thread/200-Update-1-0-5-erzeugt-Fatal-Error
Merge pull request #36 from max-m/patch-1

Add missing "use" of wcf\data\user\UserProfile