service: Add fgm services

Adds the basic skeleton for the fgm services based off the information
provided by Switch Brew.

src/core/hle/service/fgm/fgm.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::FGM {
+
+void InstallInterfaces(SM::ServiceManager& sm);
+
+} // namespace Service::FGM