as suggested 16 months ago by Gleb Naumenko in #15197 (comment).
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 in the file with it and adds the explicit include header.
Co-authored by: Gleb Naumenko naumenko.gs@gmail.com