If you open your character’s spell book from the character panel, you should see all the actions (not just spells) your character can perform and be able to drag Hide back where you want it on the tool bar. At least, that’s how I finally worked out how to add Jump back on the PC when I did the same!
Also on PC you can right click items in the inventory and choose to add them to the Custom tab. And maybe the spell book too, though haven’t tried that. Possibly you can click and drag too, but again I haven’t experimented with this yet.
