Add System.IO.MemoryMappedFiles dependency

This is a runtime dependency that causes an internal exception to be
thrown and caught when not available.

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

@@ -12,6 +12,7 @@
     "dependencies": {
         "Newtonsoft.Json": "8.0.2",
         "System.Xml.XDocument": "4.0.11-rc3-23922",
+        "System.IO.MemoryMappedFiles": "4.0.0-rc3-23922",
         "Microsoft.PowerShell.Commands.Management": "1.0.0-*",
         "Microsoft.PowerShell.Commands.Utility": "1.0.0-*"
     },