Fix regression - removed shutdown XPC inadvertently. (#675)

## Motivation and Context
#654 forgot to include the shutdown XPC that was added in #628.

Sources/Helpers/RuntimeLinux/RuntimeLinuxHelper+Start.swift

@@ -96,6 +96,7 @@ extension RuntimeLinuxHelper {
                         SandboxRoutes.wait.rawValue: server.wait,
                         SandboxRoutes.start.rawValue: server.startProcess,
                         SandboxRoutes.dial.rawValue: server.dial,
+                        SandboxRoutes.shutdown.rawValue: server.shutdown,
                     ],
                     log: log
                 )