[clang-tidy] convert several functions to const &

Found with performance-unnecessary-value-param

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/db/plugins/simple/Directory.hxx

@@ -284,8 +284,8 @@ public:
 	 * Caller must lock #db_mutex.
 	 */
 	void Walk(bool recursive, const SongFilter *match,
-		  VisitDirectory visit_directory, VisitSong visit_song,
-		  VisitPlaylist visit_playlist) const;
+		  const VisitDirectory& visit_directory, const VisitSong& visit_song,
+		  const VisitPlaylist& visit_playlist) const;
 
 	gcc_pure
 	LightDirectory Export() const noexcept;