db/simple: add option to hide CUE target songs

This reduces duplicates in the music database by hiding the original
song file when it is referenced by a CUE sheet.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1275

src/db/plugins/simple/SimpleDatabasePlugin.hxx

@@ -44,6 +44,8 @@ class SimpleDatabase : public Database {
 	bool compress;
 #endif
 
+	bool hide_playlist_targets;
+
 	/**
 	 * The path where cache files for Mount() are located.
 	 */