missing using.

src/Avalonia.Controls.DataGrid/DataGridTemplateColumn.cs

@@ -8,6 +8,7 @@ using Avalonia.Controls.Utils;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Media;
+using Avalonia.Metadata;
 using Avalonia.Utilities;
 
 namespace Avalonia.Controls