Add missing file from System.Reactive/build

Rx.NET/Source/src/System.Reactive/build/System.Reactive.net6.0-windows.targets

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+   
+  <Target Name="_RXNETWindowsTFMCheck" BeforeTargets="ResolveAssemblyReferences;Build;Rebuild">
+    <Error 
+        Text = "The 'System.Reactive' nuget package cannot be used to target '$(TargetFramework)'. Target 'net6.0-windows10.0.19041' or later instead." />
+  </Target>
+  
+</Project>