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 [[https://github.com/bitcoin/bitcoin/pull/18931 | PR18931]]