VS: Handle VS_GLOBAL_RootNamespace special case

Although we provide a `VS_GLOBAL_ROOTNAMESPACE` option to both set
the `RootNamespace` value and reference it, some users may try to
set `VS_GLOBAL_RootNamespace` to set `RootNamespace` as a variant
of the `VS_GLOBAL_<variable>` property.  In this case we still
need to add the reference to `$(RootNamespace)`.