Add Windmill to farming knowledge in the Worlds folder.

Semantically, that makes sense, but I haven't taken a look at all the maps.

planet/Worlds.ocf/Functions.ocd/System.ocg/Knowledge.c

@@ -70,7 +70,7 @@ global func GivePlayerFarmingKnowledge(int plr)
 {
 	var knowledge = [
 		// Structures needed to process farming materials.
-		Kitchen, Loom,
+		Kitchen, Loom, Windmill,
 		// Basic tools for farming.
 		Sickle,
 		// Processed goods.