gn_helper: add missing sys import

A recent change added sys usage without the corresponding import. Add it.
See https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6217699

Bug: 393209409
Change-Id: Ib30dc8938ec6e3b56ab35a7b5e2b2071cb9e84ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6300492
Auto-Submit: Tomasz Śniatowski <tomasz.sniatowski@xperi.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Philipp Wollermann <philwo@google.com>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>

gn_helper.py

@@ -5,6 +5,7 @@
 
 import os
 import re
+import sys
 
 
 def _find_root(output_dir):