gst/matroska/: Fix the Forte build by making function declaration signatures match the implementations.

Original commit message from CVS:
* gst/matroska/ebml-read.c: (gst_ebml_read_seek):
* gst/matroska/matroska-demux.c:
(gst_matroska_demux_handle_seek_event),
(gst_matroska_demux_parse_contents_seekentry),
(gst_matroska_demux_loop):
Fix the Forte build by making function declaration signatures
match the implementations.