dns/dnsdist: unbreak with boost 1.67

In file included from dnsdist.cc:44:
In file included from ./dnsdist.hh:46:
In file included from /usr/local/include/boost/uuid/uuid_generators.hpp:17:
In file included from /usr/local/include/boost/uuid/random_generator.hpp:19:
In file included from /usr/local/include/boost/tti/has_member_function.hpp:15:
In file included from /usr/local/include/boost/tti/detail/dmem_fun.hpp:11:
In file included from /usr/local/include/boost/function_types/is_member_function_pointer.hpp:14:
In file included from /usr/local/include/boost/function_types/components.hpp:61:
/usr/local/include/boost/function_types/detail/class_transform.hpp:26:31: error: expected a qualified name after 'typename'
template<typename T, typename L>

^

./dns.hh:238:11: note: expanded from macro 'L'
#define L theL()

^

PR: 227427
Reported by: antoine (via exp-run)
Obtained from: upstream