Add System.Xml.XDocument dependency to host

This resolves #604 but leaves an open question of why the package is
required at runtime, but not at compile-time.

src/Microsoft.PowerShell.Linux.Host/project.json

@@ -11,6 +11,7 @@
 
     "dependencies": {
         "Newtonsoft.Json": "8.0.2",
+        "System.Xml.XDocument": "4.0.11-rc3-23829",
         "Microsoft.PowerShell.Commands.Management": "1.0.0-*",
         "Microsoft.PowerShell.Commands.Utility": "1.0.0-*"
     },