message: core: fix systems shadowing
diff: @@ -100,7 +100,7 @@ std::string MavsdkImpl::version() const
  std::vector<std::weak_ptr<System>> MavsdkImpl::systems() const
  {
  - std::vector<std::weak_ptr<System>> systems{};
  + std::vector<std::weak_ptr<System>> systems_result{};
  std::lock_guard<std::recursive_mutex> lock(_systems_mutex);
  for (auto& system : _systems) {
  @@ -110,10 +110,10 @@ std::vector<std::weak_ptr<System>> MavsdkImpl::systems() const
  if (system.first == 0) {
  continue;
  }
  - systems.push_back(std::weak_ptr<System>(system.second));
  + systems_result.push_back(std::weak_ptr<System>(system.second));
  }
  - return systems;
  + return systems_result;
  }
  void MavsdkImpl::receive_message(mavlink_message_t& message)
  