Move 'Activity' into its own namespace in OpenRA.Game.

OpenRA.Mods.TS/Activities/VoxelHarvesterDockSequence.cs

@@ -10,6 +10,7 @@
 
 using System;
 using System.Collections.Generic;
+using OpenRA.Activities;
 using OpenRA.Mods.RA.Activities;
 using OpenRA.Mods.RA.Traits;
 using OpenRA.Mods.TS.Traits;

OpenRA.Mods.TS/Traits/Buildings/TiberianSunRefinery.cs

@@ -8,6 +8,7 @@
  */
 #endregion
 
+using OpenRA.Activities;
 using OpenRA.Mods.RA.Traits;
 using OpenRA.Mods.TS.Activities;
 using OpenRA.Traits;