Add readonly to structs

OpenRA.Game/Support/PerfSample.cs

@@ -14,7 +14,7 @@ using System.Diagnostics;
 
 namespace OpenRA.Support
 {
-	public struct PerfSample : IDisposable
+	public readonly struct PerfSample : IDisposable
 	{
 		readonly string item;
 		readonly long ticks;

OpenRA.Game/Support/VariableExpression.cs

@@ -157,7 +157,7 @@ namespace OpenRA.Support
 			Invalid = ~0
 		}
 
-		struct TokenTypeInfo
+		readonly struct TokenTypeInfo
 		{
 			public readonly string Symbol;
 			public readonly Precedence Precedence;