message: Add missing forward declarations
diff: @@ -104,7 +104,9 @@ class group;
  class group_module;
  class inbound_path;
  class ipv4_address;
  +class ipv4_subnet;
  class ipv6_address;
  +class ipv6_subnet;
  class local_actor;
  class mailbox_element;
  class message;
  @@ -161,6 +163,8 @@ enum class atom_value : uint64_t;
  // -- aliases ------------------------------------------------------------------
  using actor_id = uint64_t;
  +using ip_address = ipv6_address;
  +using ip_subnet = ipv6_subnet;
  using stream_slot = uint16_t;
  // -- functions ----------------------------------------------------------------
  