util/glsl2spriv: make --vn required

util/glsl2spriv: make --vn required

I'm not 100% sure whether it's right to make --vn required, or to avoid
the static conversion, but this seems correct. Mypy (type checking
coming soon) points out that if --vn is None then the
convert_to_static_variable function will fail. Our one use of this sets
--vn, so there is no change there. Making --vn required
ensures that it will never be None, avoiding the problem.

Reviewed-by: Luis Felipe Strano Moraes <luis.strano@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19449>