Add missing sync interfaces.

OpenRA.Game/Traits/World/Shroud.cs

@@ -20,7 +20,7 @@ namespace OpenRA.Traits
 		public object Create(ActorInitializer init) { return new Shroud(init.Self); }
 	}
 
-	public class Shroud
+	public class Shroud : ISync
 	{
 		[Sync] public bool Disabled;
 

OpenRA.Mods.Common/Effects/GravityBomb.cs

@@ -38,7 +38,7 @@ namespace OpenRA.Mods.Common.Effects
 		public IEffect Create(ProjectileArgs args) { return new GravityBomb(this, args); }
 	}
 
-	public class GravityBomb : IEffect
+	public class GravityBomb : IEffect, ISync
 	{
 		readonly GravityBombInfo info;
 		readonly Animation anim;

OpenRA.Mods.Common/Traits/ExternalCapturable.cs

@@ -49,7 +49,7 @@ namespace OpenRA.Mods.Common.Traits
 		public object Create(ActorInitializer init) { return new ExternalCapturable(init.Self, this); }
 	}
 
-	public class ExternalCapturable : ITick
+	public class ExternalCapturable : ITick, ISync
 	{
 		[Sync] public int CaptureProgressTime = 0;
 		[Sync] public Actor Captor;

OpenRA.Mods.Common/Traits/ParaDrop.cs

@@ -27,7 +27,7 @@ namespace OpenRA.Mods.Common.Traits
 		public object Create(ActorInitializer init) { return new ParaDrop(init.Self, this); }
 	}
 
-	public class ParaDrop : ITick, INotifyRemovedFromWorld
+	public class ParaDrop : ITick, ISync, INotifyRemovedFromWorld
 	{
 		readonly ParaDropInfo info;
 		readonly Actor self;