db: add compile-time option to disable database

src/Partition.hxx

@@ -79,10 +79,14 @@ struct Partition {
 		return playlist.DeleteRange(pc, start, end);
 	}
 
+#ifdef ENABLE_DATABASE
+
 	void DeleteSong(const char *uri) {
 		playlist.DeleteSong(pc, uri);
 	}
 
+#endif
+
 	void Shuffle(unsigned start, unsigned end) {
 		playlist.Shuffle(pc, start, end);
 	}