Add missing using System.Linq to fix compile error

Co-authored-by: emako <24737061+emako@users.noreply.github.com>

QuickLook.Plugin/QuickLook.Plugin.MarkdownViewer/MarkdownPanel.cs

@@ -23,6 +23,7 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Globalization;
 using System.IO;
+using System.Linq;
 using System.Reflection;
 using System.Text;
 using UtfUnknown;