Add missing usings

samples/BindingDemo/App.xaml.cs

@@ -3,6 +3,7 @@ using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Logging.Serilog;
 using Avalonia.Markup.Xaml;
+using Avalonia.ReactiveUI;
 using Serilog;
 
 namespace BindingDemo

samples/ControlCatalog.Desktop/Program.cs

@@ -4,6 +4,7 @@ using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Logging.Serilog;
 using Avalonia.Platform;
+using Avalonia.ReactiveUI;
 using Serilog;
 
 namespace ControlCatalog

samples/ControlCatalog.NetCore/Program.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using System.Threading;
 using Avalonia;
 using Avalonia.Skia;
+using Avalonia.ReactiveUI;
 
 namespace ControlCatalog.NetCore
 {

samples/RenderDemo/App.xaml.cs

@@ -4,6 +4,7 @@
 using Avalonia;
 using Avalonia.Logging.Serilog;
 using Avalonia.Markup.Xaml;
+using Avalonia.ReactiveUI;
 
 namespace RenderDemo
 {

samples/VirtualizationDemo/Program.cs

@@ -5,6 +5,7 @@ using System;
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Logging.Serilog;
+using Avalonia.ReactiveUI;
 using Serilog;
 
 namespace VirtualizationDemo