Rocket/BFG are missing ammo icons

[XaserAcheron]

Whoops! :P

Thinking on it, I don't think I ever made a graphic for BFG Cells. Better fix that. :P

[XaserAcheron]

So... even though this wasn't a zscript thing, I lazily fixed it in the zscript branch since it was active so it'll hit master here in a bit. :P