[video_player] Add missing mutex (#24)

packages/video_player/elinux/gst_video_player.h

@@ -66,6 +66,7 @@ class GstVideoPlayer {
   bool mute_ = false;
   bool auto_repeat_ = false;
   bool is_completed_ = false;
+  std::mutex mutex_event_completed_;
   std::shared_mutex mutex_buffer_;
   std::unique_ptr<VideoPlayerStreamHandler> stream_handler_;
 };