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