[autotest] fix error |ImportError: No module named autotest_lib.client.common_lib|

import common should be added for the script to run on devserver.

BUG=None
TEST=None

Change-Id: Ie03f7af5dd70cf190969897fc871df3fb96fe264
Reviewed-on: https://chromium-review.googlesource.com/185608
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Alex Miller <milleral@chromium.org>
diff --git a/site_utils/admin/clean_staged_images.py b/site_utils/admin/clean_staged_images.py
index 59cdedb..32aee98 100755
--- a/site_utils/admin/clean_staged_images.py
+++ b/site_utils/admin/clean_staged_images.py
@@ -28,6 +28,7 @@
 import shutil
 import time
 
+import common
 from autotest_lib.client.common_lib import global_config
 
 # This filename must be kept in sync with devserver's downloader.py