[refactor] remove #include <validation.h> from zmqnotificationinterface.cpp

Summary:
upcoming PR16443 adds an #include <txdb.h> to validation.h which makes
zmq want leveldb symbols. this is the first step towards avoiding that and
a generally Good Thing To Do. #include <primitives/block.h> will do.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7550