Fix "Deprecated: Creation of dynamic property" (#531)

Deprecated: Creation of dynamic property DynamicHLSPlaylist::$playlistResolver is deprecated in file on line 94

Author: marcialpaulg

src/Http/DynamicHLSPlaylist.php

@@ -35,6 +35,13 @@ class DynamicHLSPlaylist implements Responsable
      */
     private $mediaResolver;
 
+    /**
+     * Callable to retrieve the path to the given playlist.
+     *
+     * @var callable
+     */
+    private $playlistResolver;
+
     /**
      * @var array
      */