Pass random_exlude_pattern as reference

src/mpdpp.cpp

@@ -602,7 +602,7 @@ bool Connection::AddRandomTag(mpd_tag_type tag, size_t number, std::mt19937 &rng
 	return true;
 }
 
-bool Connection::AddRandomSongs(size_t number, std::string random_exclude_pattern, std::mt19937 &rng)
+bool Connection::AddRandomSongs(size_t number, const std::string &random_exclude_pattern, std::mt19937 &rng)
 {
 	prechecksNoCommandsList();
 	std::vector<std::string> files;

src/mpdpp.h

@@ -547,7 +547,7 @@ struct Connection
 	int AddSong(const std::string &, int = -1); // returns id of added song
 	int AddSong(const Song &, int = -1); // returns id of added song
 	bool AddRandomTag(mpd_tag_type, size_t, std::mt19937 &rng);
-	bool AddRandomSongs(size_t number, std::string random_exclude_pattern, std::mt19937 &rng);
+	bool AddRandomSongs(size_t number, const std::string &random_exclude_pattern, std::mt19937 &rng);
 	void Add(const std::string &path);
 	void Delete(unsigned int pos);
 	void PlaylistDelete(const std::string &playlist, unsigned int pos);