www/newsboat: unbreak with libc++ 7

In file included from src/ttrss_api.cpp:1:
In file included from include/ttrss_api.h:4:
./3rd-party/json.hpp:1201:9: error: static_assert failed "could not find from_json() method in T's namespace"

static_assert(sizeof(BasicJsonType) == 0,
^             ~~~~~~~~~~~~~~~~~~~~~~~~~~

./3rd-party/json.hpp:1216:16: note: in instantiation of function template specialization 'nlohmann::detail::from_json_fn::call<nlohmann::basic_json<std::map, std::vector, std::1::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer>, std::1::basic_string_view<char, std::__1::char_traits<char> > >' requested here

return call(j, val, priority_tag<1> {});
       ^

./3rd-party/json.hpp:9660:9: note: in instantiation of function template specialization 'nlohmann::detail::from_json_fn::operator()<nlohmann::basic_json<std::map, std::vector, std::1::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer>, std::1::basic_string_view<char, std::__1::char_traits<char> > >' requested here

::nlohmann::from_json(std::forward<BasicJsonType>(j), val);
^

./3rd-party/json.hpp:12181:36: note: in instantiation of function template specialization 'nlohmann::adl_serializer<std::1::basic_string_view<char, std::1::char_traits<char> >, void>::from_json<const nlohmann::basic_json<std::map, std::vector, std::1::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer> &, std::1::basic_string_view<char, std::__1::char_traits<char> > >' requested here

JSONSerializer<ValueType>::from_json(*this, ret);
                           ^

./3rd-party/json.hpp:12443:16: note: in instantiation of function template specialization 'nlohmann::basic_json<std::map, std::vector, std::1::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer>::get<std::1::basic_string_view<char, std::1::char_traits<char> >, std::1::basic_string_view<char, std::__1::char_traits<char> >, 0>' requested here

return get<ValueType>();
       ^

/usr/include/c++/v1/string:875:29: note: in instantiation of function template specialization 'nlohmann::basic_json<std::map, std::vector, std::1::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer>::operator basic_string_view<std::1::basic_string_view<char, std::__1::char_traits<char> >, 0>' requested here

{__self_view __sv = __t; return assign(__sv);}
                    ^

src/ttrss_api.cpp:69:7: note: in instantiation of function template specialization 'std::1::basic_string<char, std::1::char_traits<char>, std::1::allocator<char> >::operator=<nlohmann::basic_json<std::map, std::vector, std::1::basic_string<char>, bool, long, unsigned long, double, std::allocator, adl_serializer>, void>' requested here

sid = content["session_id"];
    ^

PR: 230631
Reported by: antoine (via exp-run)
Obtained from: upstream (json)
Approved by: portmgr blanket