Fixed the sietch crashing the game when placed on a map

Noticed during testing of https://github.com/OpenRA/OpenRA/pull/4496. It looks like bit rot as the `Sprite` field does not exist anymore on https://github.com/OpenRA/OpenRA/wiki/Traits#bib. Not sure why OpenRA.Lint did not pick this up. https://github.com/OpenRA/OpenRA/blob/bleed/OpenRA.Game/GameRules/ActorInfo.cs#L81