Ensure Umbraco Razor files are compiled into <project>.Views.dll

It worked properly if the Umbraco files are on disk before a build
but when the files are not there yet or a dotnet clean is done first the
files are not compiled. This is often the case on build servers that start
with a clean working directory for each build.

Using this target we simply ensure the files are included as "Content"
which Razor will scan for .cshtml files in their "ResolveRazorGenerateInputs"
target.

build/NuSpecs/buildTransitive/Umbraco.Cms.StaticAssets.targets

@@ -87,4 +87,10 @@
         <RemoveDir Directories="@(WwwrootUmbracoPackageDir)"  />
     </Target>
 
+    <Target Name="IncludeUmbracoRazorFiles" BeforeTargets="ResolveRazorGenerateInputs">
+        <ItemGroup>
+            <Content Include="$(MSBuildProjectDirectory)\umbraco\**\*.cshtml" />
+        </ItemGroup>
+    </Target>
+
 </Project>