LibCore: Add Promise to the forward declare header

Author: https://github.com/kleinesfilmroellchen
Commit: afd0f941b7
Pull-request: https://github.com/SerenityOS/serenity/pull/16715
Issue: https://github.com/SerenityOS/serenity/issues/14945
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/BenWiederhake

Userland/Libraries/LibCore/Forward.h

@@ -34,6 +34,8 @@ class Object;
 class ObjectClassRegistration;
 class ProcessStatisticsReader;
 class Socket;
+template<typename Result>
+class Promise;
 class SocketAddress;
 class TCPServer;
 class TCPSocket;