Add the missing 'import stat' line.

Accidentally broken by the cherry-pick in commit
f829b40c48 - the original CL in oc-dev
doesn't require the 'import stat' line.

Bug: 63629728
Test: `pylint --rcfile=pylintrc sign_target_files_apks.py`
Test: Successfully sign a build with sign_target_files_apks.py.
Change-Id: I94be613fb2219597148c4339ac33fc93d0cdae47

tools/releasetools/sign_target_files_apks.py

@@ -102,6 +102,7 @@ import copy
 import errno
 import os
 import re
+import stat
 import subprocess
 import tempfile
 import zipfile