bird2: document the OSPF alignment patch

bird2: document the OSPF alignment patch

The patch carried only a subject line and a placeholder commit id, so
nothing recorded why marking struct top_hash_entry as PACKED fixes a
bus error, or whether it can be dropped.

Restore the description and the sign-off from the commit that
introduced it in the routing feed, keeping the original author's
wording, and record the upstream status. Verified that BIRD 2.19.2
still declares the structure without any alignment attribute, so the
patch is still needed.

The header keeps the "---" separator the sibling patch uses, and
orders Signed-off-by last among the trailers.

Signed-off-by: Josef Schlehofer <redacted>
Co-authored-by: Claude Fable 5 <redacted>