Fixes: https://github.com/xamarin/xamarin-macios/issues/8265
Usage of `System.Drawing` types in iOS binding projects are failing with:
error CS1069: The type name 'Color' could not be found in the namespace 'System.Drawing'.
This type has been forwarded to assembly 'System.Drawing.Common, Version=4.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'
Consider adding a reference to that assembly.
In 3a7bdc0, an `_AddExtraReferences` MSBuild target was added for iOS
and Mac projects, but mistakenly missing from binding projects.
I moved the `_AddExtraReferences` target to `Xamarin.Shared.targets`,
so it will be used for all project types.
I also updated a test, I could merely use `System.Drawing.Color` in
`StructsAndEnums.cs` to reproduce the failure.
666059f289