libfilezilla: try requiring gcc 8

to fix
nvoker.cpp: In function 'fz::invoker_factory fz::get_invoker_factory(fz::event_loop&)':
invoker.cpp:28:54: error: 'loop' was not declared in this scope
  return [handler = std::optional<thread_invoker>(), &loop](std::function<void()> const& cb) mutable {
                                                      ^~~~
invoker.cpp: In lambda function:
invoker.cpp:30:20: error: 'loop' is not captured
    handler.emplace(loop);
                    ^~~~
invoker.cpp:28:58: note: the lambda has no capture-default
  return [handler = std::optional<thread_invoker>(), &loop](std::function<void()> const& cb) mutable {
                                                          ^
invoker.cpp:28:54: note: '<typeprefixerror>loop' declared here
  return [handler = std::optional<thread_invoker>(), &loop](std::function<void()> const& cb) mutable {
                                                      ^~~~
seen with gcc 7 on NetBSD 9.


(wiz)