rtmp2src: Set buffer timestamp on output buffer

This timestamp information would be useful for queue2 element
when calculating time level and also it makes buffering decision
more reliable.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1727>

gst/rtmp2/gstrtmp2src.c

@@ -680,6 +680,8 @@ gst_rtmp2_src_create (GstBaseSrc * src, guint64 offset, guint size,
     self->sent_header = TRUE;
   }
 
+  GST_BUFFER_DTS (buffer) = self->last_ts;
+
   *outbuf = buffer;
 
   gst_buffer_unref (message);