message: Fix include and cast with socket to int conversion
diff: #pragma once
  -#include <cstddef>
  +#include <cstdint>
  #include "caf/config.hpp"
  @@ -30,7 +30,7 @@ namespace network {
  using native_socket = size_t;
  constexpr native_socket invalid_native_socket = static_cast<native_socket>(-1);
  inline int64_t int64_from_native_socket(native_socket sock) {
  - return sock == invalid_native_socket ? -1 : static_cast<uint64_t>(sock);
  + return sock == invalid_native_socket ? -1 : static_cast<int64_t>(sock);
  }
  #else
  using native_socket = int;
  