docs: add since markers to new codecparsers video meta API

https://bugzilla.gnome.org/show_bug.cgi?id=691712

gst-libs/gst/codecparsers/gstmpegvideometa.c

@@ -102,6 +102,8 @@ gst_mpeg_video_meta_get_info (void)
  * Provided structures must either be %NULL or GSlice-allocated.
  *
  * Returns: (transfer full): a newly created #GstMpegVideoMeta
+ *
+ * Since: 1.2
  */
 GstMpegVideoMeta *
 gst_buffer_add_mpeg_video_meta (GstBuffer * buffer,

gst-libs/gst/codecparsers/gstmpegvideometa.h

@@ -55,6 +55,8 @@ const GstMetaInfo * gst_mpeg_video_meta_get_info (void);
  *
  * The various fields are only valid during the lifetime of the #GstH264Meta.
  * If elements wish to use those for longer, they are required to make a copy.
+ *
+ * Since: 1.2
  */
 struct _GstMpegVideoMeta {
   GstMeta            meta;