hlssink: Add segment target-duration property

This property causes hlssink to push a force key unit event upstream
every target-duration seconds. This is for simple automatic segmenting.
To disable it, set the property to 0 and push the force key unit event
manually.
diff --git a/gst/hls/gsthlssink.h b/gst/hls/gsthlssink.h
index 44c60da..e678e0c 100644
--- a/gst/hls/gsthlssink.h
+++ b/gst/hls/gsthlssink.h
@@ -50,6 +50,9 @@
   GstM3U8Playlist *playlist;
   guint index;
   gint max_files;
+  gint target_duration;
+  gint count;
+  guint timeout_id;
 };
 
 struct _GstHlsSinkClass