playlist: move PLAYLIST_COMMENT to stored_playlist.c

Only used there.

src/playlist.h

@@ -24,8 +24,6 @@
 
 #include <stdbool.h>
 
-#define PLAYLIST_COMMENT	'#'
-
 struct player_control;
 
 enum playlist_result {

src/stored_playlist.c

@@ -37,6 +37,8 @@
 #include <string.h>
 #include <errno.h>
 
+static const char PLAYLIST_COMMENT = '#';
+
 static unsigned playlist_max_length;
 bool playlist_saveAbsolutePaths = DEFAULT_PLAYLIST_SAVE_ABSOLUTE_PATHS;