Is that a crack or a chip? If it is a crack, get some aquarium silicon sealant and put it on there, then re-install the bulkhead before it dries. If it is a chip, it might be worth doing the same thing because it might not allow your bulkhead gasket to seal properly.
Looks like a chip with a crack forming from the hole. I would reinstall the bulkhead with a liberal amount of silicone on the inside to from a seal around the chip and crack, then monitor the bulkhead closely for leaks as you move forward. It will fail eventually, whether that be months or years from now, but you're just gonna have to keep an eye on it as long as the tank is in use.
Okay, maybe I'm paranoid, but...if that's just a chip, I would personally remove the bulkhead, get a piece of glass large enough to act as a patch and silicone it securely in place covering the entire hole, the chip and at least a couple inches of the surrounding glass. The bulkhead would either be re-drilled and relocated, or perhaps drilled into the patch and allowed to go through that same hole if the threaded shaft is long enough.
If that little line is actually a crack, rather than just a "hair" of dried silicone pulled off during the initial installation, then I wouldn't even consider using the tank. A crack will eventually spread and result in failure, quite possibly catastrophic.
On a closer look at that picture, I think it is two chips on top of each other with no crack. I think following jjohnwm's advice of siliconing a small piece of glass over it is a better idea than just filling it with silicon. It actually can be pretty thin glass because it's only purpose is to give a flat surface for the bulkhead to seal on. Get a couple small 4" x 4 "scraps of double pane window glass and drill it to the same size as the hole. The extras are in case the window glass cracks while you drill it, and you might be better off with slighly larger pieces to be able to clamp it better while drilling.
One more thought: if the chips are on the outside of the glass you don't have to do anything. The bulkhead will hold with no additional work.