Ensure SpawnStartingUnits initializes after Locomotor.

Prior to ef44c31, Locomotor would be earlier in the trait initialization sequence than SpawnStartingUnits. After this commit, the initialization sequence was perturbed and SpawnStartingUnits would initialize first. When SpawnStartingUnits would query CanEnterCell this would generate a null reference as Locomotor had not yet initialized.

SpawnStartingUnitsInfo is made to initialize NotBefore LocomotorInfo to enforce the required trait ordering.

OpenRA.Mods.Common/Traits/World/SpawnStartingUnits.cs

@@ -20,7 +20,7 @@ namespace OpenRA.Mods.Common.Traits
 {
 	[TraitLocation(SystemActors.World)]
 	[Desc("Spawn base actor at the spawnpoint and support units in an annulus around the base actor. Both are defined at MPStartUnits. Attach this to the world actor.")]
-	public class SpawnStartingUnitsInfo : TraitInfo, Requires<StartingUnitsInfo>, ILobbyOptions
+	public class SpawnStartingUnitsInfo : TraitInfo, Requires<StartingUnitsInfo>, NotBefore<LocomotorInfo>, ILobbyOptions
 	{
 		public readonly string StartingUnitsClass = "none";