json_spirit: avoid using bind placeholders in global namespace

json_spirit: avoid using bind placeholders in global namespace

to silence the warning from boost v1.73, like

json_spirit/json_spirit_reader.cpp:7:
/opt/ceph/include/boost/bind.hpp:36:1: note: ‘#pragma message: The practice of declaring the Bind placeholders (_1, _2, ...) in the global namespace is deprecated. Please use <boost/bind/bind.hpp> +
using namespace boost::placeholders, or define BOOST_BIND_GLOBAL_PLACEHOLDERS to retain the current behavior.’
   36 | BOOST_PRAGMA_MESSAGE(
      | ^~~~~~~~~~~~~~~~~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>