service: Add migration services

Adds the basic skeleton for the mig:usr service based off information
provided by Switch Brew.

src/core/hle/service/mig/mig.h

@@ -0,0 +1,15 @@
+// Copyright 2018 yuzu emulator team
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+namespace Service::SM {
+class ServiceManager;
+}
+
+namespace Service::Migration {
+
+void InstallInterfaces(SM::ServiceManager& sm);
+
+} // namespace Service::Migration