[siso]Adding import

`shutil` was used in
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6420348
but not imported, leading to error message when executing
_is_google_corp_machine

Bug: None
Change-Id: I0bec8986f531a31905dabaadd20af68c1cd5536f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6449462
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Junji Watanabe <jwata@google.com>
Auto-Submit: Arthur Milchior <arthurmilchior@chromium.org>

siso.py

@@ -9,6 +9,7 @@ binary when run inside a gclient source tree, so users can just type
 
 import os
 import signal
+import shutil
 import subprocess
 import sys