fix warnings about BOOST deprecated header

make[2]: Wejście do katalogu '/home/godfryd/isc/repos/aaa/kea-1.6.0-beta-git/src/lib/util'
  CXX      boost_time_utils.lo
  CXX      csv_file.lo
  CXX      filename.lo
  CXX      labeled_value.lo
  CXX      memory_segment_local.lo
  CXX      pid_file.lo
  CXX      process_spawn.lo
  CXX      state_model.lo
  CXX      signal_set.lo
  CXX      stopwatch.lo
  CXX      stopwatch_impl.lo
  CXX      strutil.lo
  CXX      time_utilities.lo
  CXX      watch_socket.lo
  CXX      versioned_csv_file.lo
  CXX      encode/base_n.lo
  CXX      random/qid_gen.lo
In file included from /usr/include/boost/config/header_deprecated.hpp:18,
                 from /usr/include/boost/math/common_factor_ct.hpp:14,
                 from /usr/include/boost/math/common_factor.hpp:13,
                 from encode/base_n.cc:21:
/usr/include/boost/math/common_factor_ct.hpp:16:1: note: #pragma message: This header is deprecated. Use <boost/integer/common_factor_ct.hpp> instead.
   16 | BOOST_HEADER_DEPRECATED("<boost/integer/common_factor_ct.hpp>");
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/math/common_factor_rt.hpp:13:1: note: #pragma message: This header is deprecated. Use <boost/integer/common_factor_rt.hpp> instead.
   13 | BOOST_HEADER_DEPRECATED("<boost/integer/common_factor_rt.hpp>");
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/boost/math/common_factor.hpp:16:1: note: #pragma message: This header is deprecated. Use <boost/integer/common_factor.hpp> instead.
   16 | BOOST_HEADER_DEPRECATED("<boost/integer/common_factor.hpp>");
      | ^~~~~~~~~~~~~~~~~~~~~~~