glsl/xxd.py: fix imports

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

fixes: 412472da5c
       ("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>

src/compiler/glsl/xxd.py

@@ -25,8 +25,7 @@
 from __future__ import unicode_literals
 import argparse
 import io
-import string
-import sys
+import os
 
 
 def get_args():