fixup! feat: Extensibility 0.4.0 for GE6

Author: gerhardol

src/GitExtensions.PluginManager/Plugin.cs

@@ -7,6 +7,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Composition;
 using System.Diagnostics;
 using System.IO;
 using System.Windows.Forms;