gst/matroska/: Set timestamps on outgoing ebml headers as well, so that the element after matroskamux can get the tim...

Original commit message from CVS:
Reviewed by: Tim-Philipp Müller  <tim at centricular dot net>
* gst/matroska/ebml-write.c: (gst_ebml_write_new),
(gst_ebml_write_reset), (gst_ebml_write_element_new):
* gst/matroska/ebml-write.h:
* gst/matroska/matroska-mux.c: (gst_matroska_mux_write_data):
Set timestamps on outgoing ebml headers as well, so that the
element after matroskamux can get the timestamp already when
reading the first ebml element and doesn't have to wait for
the actual data buffer for that (#320308).