glsl/xxd.py: fix imports

sys and string are unused, os is needed but not imported

fixes: 412472da5cb30b603e218b34893936cc70039ded
       ("glsl: Add utility to convert text files to C strings")

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7034>
1 file changed