Fix outdated type definitions for testing on Unity side

Author: hadashiA

src/MemoryPack.Unity/Assets/Tests/MemoryPackObjects/MemberKinds.cs

@@ -16,9 +16,9 @@ public partial class MemberKindsAllUnmanaged
         public int A; // public field
         [MemoryPackInclude]
         private int B; // private field
-        // public readonly int C; // public readonly field
-        // [MemoryPackInclude]
-        // private readonly int D; // readonly field
+        public readonly int C; // public readonly field
+        [MemoryPackInclude]
+        private readonly int D; // readonly field
 
         public int E { get; set; } // public property
         public int F { private get; set; } // private get