shells/powershell: Install the lib/powershell/ref symlink

This fixes a error that happens when typing

Add-Type -TypeDefinition @"
using System;
"@ -Language CSharp

into PowerShell's prompt.

It is yet unclear if all .NET DLLs should actually be installed under /ref,
but this is still better than non-working Add-Type command.

Reported by: Luke Brennan <lukebrennan@outlook.com>