vl/bitstream: use an int32_t for se encoding.

This seems to fix a bug found with radv and ffmpeg encoding

Fixes: 1782ab4d8bab ("util: add a bitstream encoder for video stream headers.")
Acked-by: Hyunjun Ko <zzoon@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31086>