Add missing includes in assumptions.h and chainstate.cpp

Summary:
The inclusion of this header should not depend on the inclusion of other
headers that include cstddef themselves.

This is a partial backport of core#27254
https://github.com/bitcoin/bitcoin/pull/27254/commits/106b46d9d25b5228ef009fbbe6f9a7ae35090d15
https://github.com/bitcoin/bitcoin/pull/27254/commits/b202b3dd6393b415fa68e18dc49c9431dc6b58b2

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16153