Fix preprocessor directive placement in policy class

Moved the UNITY_2023_2_OR_NEWER preprocessor directive to wrap the entire file instead of just the class, ensuring the file is only included for compatible Unity versions.

Author: mackysoft

Assets/MackySoft/MackySoft.SerializeReferenceExtensions/Editor/Unity_2023_2_OrNewer_GenericVarianceTypeCompatibilityPolicy.cs

@@ -1,9 +1,9 @@
-using System;
+#if UNITY_2023_2_OR_NEWER
+using System;
 using System.Reflection;
 
 namespace MackySoft.SerializeReferenceExtensions.Editor
 {
-#if UNITY_2023_2_OR_NEWER
     public sealed class Unity_2023_2_OrNewer_GenericVarianceTypeCompatibilityPolicy : ITypeCompatibilityPolicy
     {
 
@@ -124,5 +124,5 @@ private static bool AreAllGenericArgumentsCompatible (Type[] genericTypeParamete
             return true;
         }
     }
-#endif
-}
+}
+#endif