codecparsers: h264: recognize SVC NAL units.

Identify SVC NAL units and tag them as such. This is necessary for
gst_h264_parser_parse_slice_hdr() to fail gracefully, if the user
did not perform the check himself.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
Signed-off-by: Zhong Cong <congx.zhong@intel.com>