Fix @generated marking in Phab for generate-seeds.py

Summary: This ensures Phabricator doesn't see the file as automatically generated.

Test Plan:

python3 generate-seeds.py . > ../../src/chainparamsseeds.h

No differences should be generated.

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5793