Add .gitignore for dns seeds

Summary:
Similar to https://github.com/bitcoin/bitcoin/pull/20237/commits/36e875b4c5f0812868e34beba7be4aa1f7a00773
but include seeds_test.txt since our release process explicitly includes both.

Test Plan:

cp ~/seeds_*.txt .
git status

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12142