releasetools: Fix the missing 'post-build' in full OTAs.

The line was unintentionally removed in commit 7f804ba.

Test: ota_from_target_files.py generates a full OTA. Check the package
      metadata.

Change-Id: Icae88e2a9bb2bfc450a3d0d7ab524d6a6eac9df5
1 file changed