Add PrivateAssets="All" to package reference

Author: bording

src/Directory.Build.props

@@ -39,6 +39,6 @@
     </ItemGroup>
 
     <ItemGroup Condition=" '$(TargetFramework)' == 'net472' ">
-        <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" />
+        <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="All" />
     </ItemGroup>
 </Project>