AsmDiff: RequiresPreviewAttribute not getting auto added

[carlossanlop]

Example: dotnet/core#9176 (comment)

https://github.com/dotnet/runtime/blob/be5694d8e8494f533bfe5d48d987f5e050df80d2/src/libraries/System.Runtime/ref/System.Runtime.cs#L15088

+    public sealed class Lock {

Should have been

+    [System.Runtime.Versioning.RequiresPreviewFeaturesAttribute]
+    public sealed class Lock {