Capcom released an official patch for Street Fighter X Tekken on PC, which includes various bug fixes, balance changes, and... character unlocks !
Some users reported that Patch 1.02 (not 108 specifically, but close!) or later versions unlock all characters. You can try updating your game to the latest version. street fighter x tekken pc patch 108 character unlock link
Keep in mind that some of these links or tools might not be up-to-date or supported by Capcom. Be cautious when downloading or using third-party patches. Capcom released an official patch for Street Fighter