db/Plugin: use std::unique_ptr<> to manage Database pointers

src/db/DatabaseGlue.cxx

@@ -1,5 +1,5 @@
 /*
- * Copyright 2003-2018 The Music Player Daemon Project
+ * Copyright 2003-2019 The Music Player Daemon Project
  * http://www.musicpd.org
  *
  * This program is free software; you can redistribute it and/or modify
@@ -18,13 +18,14 @@
  */
 
 #include "DatabaseGlue.hxx"
+#include "Interface.hxx"
 #include "Registry.hxx"
 #include "DatabaseError.hxx"
 #include "util/RuntimeError.hxx"
 #include "config/Block.hxx"
 #include "DatabasePlugin.hxx"
 
-Database *
+DatabasePtr
 DatabaseGlobalInit(EventLoop &main_event_loop,
 		   EventLoop &io_event_loop,
 		   DatabaseListener &listener,