cargo: support LST

OpenRa.Game/Traits/Activities/UnloadCargo.cs

@@ -12,7 +12,8 @@ namespace OpenRa.Game.Traits.Activities
 
 		int2? ChooseExitTile(Actor self)
 		{
-			if (!Game.IsCellBuildable(self.Location, UnitMovementType.Foot, self))
+			// is anyone still hogging this tile?
+			if (Game.UnitInfluence.GetUnitsAt(self.Location).Count() > 1)
 				return null;
 
 			for (var i = -1; i < 2; i++)