Fix missing using System directive for StringComparison

Co-authored-by: captainsafia <[email protected]>

Author: Copilot

src/OpenApi/gen/XmlCommentGenerator.Parser.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Threading;