[seeder] Bump thread stacksize

Summary:
0x20000 is too small on some systems, causing pthread_create() to fail. It turns out that predicting the right value is not trivial, so we can just bump it slightly instead.

Depends on D17348, but only to discover the issue in the first place.

Test Plan:
Supposedly setting ulimit -s or ulimit -Hs would allow you to reproduce, but this does not appear to work. If you have a borked environment that is enforcing large stack sizes, just run the seeder:

./bitcoin-seeder

Without the patch it can break. With the patch, it works.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien, redball4az

Differential Revision: https://reviews.bitcoinabc.org/D17349