MFH: r431820

dns/dnsdist: unbreak build with clang 4.0

ext/json11/json11.cpp:153:24: error: invalid operands to binary expression

('nullptr_t' and 'nullptr_t')
  return m_value < static_cast<const Value<tag, T> *>(other)->m_value;
         ~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

ext/json11/json11.cpp:209:5: note: in instantiation of member function

  'json11::Value<json11::Json::Type::NUL, nullptr_t>::less' requested here
JsonNull() : Value(nullptr) {}
^

PR: 216069
Reported by: jbeich
Obtained from: upstream

Approved by: ports-secteam (blanket)