Fix reference assembly generation

When we switched to letting the new NuGetizer inference control
the package content, we missed adding the .csproj that generates
the reference assembly manually, since it's not added by default
as a Content item by the common targets.

Fixes NuGet/Home#5465

Author: kzu

src/Build/NuGet.Build.Packaging.Tasks/NuGet.Build.Packaging.Tasks.csproj

@@ -18,6 +18,9 @@
   </ItemGroup>
   <ItemGroup>
     <None Remove="**\*.props;**\*.targets" />
+    <Content Include="GenerateReferenceAssembly.csproj">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
     <None Include="NuGet.Build.Packaging.Tasks.targets" />
     <Content Include="**\*.props;**\*.targets" Exclude="obj\**\*.*;bin\**\*.*;NuGet.Build.Packaging.Tasks.targets">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>