Add the missing 'import stat' line.

Accidentally broken by the cherry-pick in commit
f829b40c489781aba8fcd2aeb8c7900f7406e695 - 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
1 file changed