Add missing reference to NETStandard.Library.NETFramework

Author: Nate McMaster

samples/Config.CustomConfigurationProviders.Sample/Config.CustomConfigurationProviders.Sample.csproj

@@ -14,4 +14,8 @@
  <PackageReference Include="System.Collections.Immutable" Version="$(ImmutableCollectionsVersion)" />
  </ItemGroup>
 
+ <ItemGroup Condition=" '$(TargetFramework)' == 'net461' ">
+ <PackageReference Include="NETStandard.Library.NETFramework" Version="$(NETStandardLibraryNETFrameworkVersion)" PrivateAssets="All" />
+ </ItemGroup>
+
 </Project>