message: Add missing export macro
diff: #include "caf/byte.hpp"
  #include "caf/byte_buffer.hpp"
  +#include "caf/detail/net_export.hpp"
  #include "caf/dictionary.hpp"
  #include "caf/string_view.hpp"
  @@ -15,7 +16,7 @@ namespace caf::net::web_socket {
  /// Wraps state and algorithms for the WebSocket client handshake as defined in
  /// RFC 6455.
  -class handshake {
  +class CAF_NET_EXPORT handshake {
  public:
  // -- member types -----------------------------------------------------------
  