message: Add midding CAF_NET_EXPORT
diff: #include <string>
  #include "caf/actor.hpp"
  +#include "caf/detail/net_export.hpp"
  #include "caf/detail/serialized_size.hpp"
  #include "caf/fwd.hpp"
  #include "caf/intrusive/drr_queue.hpp"
  namespace caf::net {
  -class endpoint_manager_queue {
  +class CAF_NET_EXPORT endpoint_manager_queue {
  public:
  enum class element_type { event, message };
  