Reference Rx.props for interop samples.

Instead of including the `PackageReference` directly in the csproj.

samples/interop/GtkInteropDemo/GtkInteropDemo.csproj

@@ -149,11 +149,7 @@
       <Name>ControlCatalog</Name>
     </ProjectReference>
   </ItemGroup>
-  <ItemGroup>
-    <PackageReference Include="System.Reactive">
-      <Version>3.0.0</Version>
-    </PackageReference>
-  </ItemGroup>
+  <Import Project="..\..\..\build\Rx.props" />
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
   <Import Project="$(MSBuildThisFileDirectory)..\..\..\src\Shared\nuget.workaround.targets" />
 </Project>

samples/interop/WindowsInteropTest/WindowsInteropTest.csproj

@@ -179,11 +179,7 @@
       <Generator>MSBuild:Compile</Generator>
     </Page>
   </ItemGroup>
-  <ItemGroup>
-    <PackageReference Include="System.Reactive">
-      <Version>3.0.0</Version>
-    </PackageReference>
-  </ItemGroup>
+  <Import Project="..\..\..\build\Rx.props" />
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
   <Import Project="$(MSBuildThisFileDirectory)..\..\..\src\Shared\nuget.workaround.targets" />
 </Project>