って is a casual form of the "quotation" particle と. しろ is the imperative form of the verb する. So the two are completely separate in terms of conjugation (i.e. しろって is not a "form" of the verb する, but a verb with a particle attached). The reason they are adjacent in the sentence is because someone is asking what someone said, so the quotation particle is needed to show that someone is saying something. In most cases, と or って with the verb left out almost always implies 言う as the verb being left out (e.g. 何をしろって言った is the implication here), but in most cases you just leave that out since it's obvious what is being implied.