backfiller: Add missing 'extra_header' field.
This field wasn't backfilled; and it wasn't caught by tests, because revisions in swh.journal.tests.journal_data.TEST_OBJECTS were had an empty extra_header field.
Starting with swh-journal v0.4.3, one of the revisions in TEST_OBJECTS has a non-empty extra_header field, so it will become a test failure without this commit
Migrated from D3807 (view on Phabricator)