releasetools: ota_from_target_files uses target_files_diff.

The dependency was overlooked as the module was conditionally imported.

Bug: 63866463
Test: `m -j ota_from_target_files`; Run ota_from_target_files with
      `--log_diff` flag (on a non-A/B target).
Change-Id: I71eff2e55cc28fd649f0ca1b38da53907dac6597