libretro: Add compatibility settings loading

src/frontend-common/game_settings.h

@@ -82,6 +82,8 @@ struct Entry
   void ApplySettings(bool display_osd_messages) const;
 };
 
+#ifndef LIBRETRO
+
 class Database
 {
 public:
@@ -97,4 +99,6 @@ private:
   std::unordered_map<std::string, Entry> m_entries;
 };
 
+#endif
+
 }; // namespace GameSettings