net: use CMessageHeader::HEADER_SIZE, add missing include

Summary:
static constexpr CMessageHeader::HEADER_SIZE is already used in this file,
src/net.cpp, in 2 instances. This commit replaces the remaining 2 integer
values with it and adds the explicit include header.

Co-authored by: Gleb Naumenko <naumenko.gs@gmail.com>

This is a backport of Core PR18931

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9094