Update Bind.hs

Add the case when the first element of the first declrs tuple is Nothing
 into extractCompType's case matching.

I'm not sure if this change is correct, but at least it resolves a
 'c2hs: src/C2HS/Gen/Bind.hs:(1610,8)-(1614,73): Non-exhaustive patterns in case'
 error which I met...