LibCore: Forward declare MappedFile

Author: https://github.com/LucasChollet
Commit: 0f4a8731fd
Pull-request: https://github.com/SerenityOS/serenity/pull/17023
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibCore/Forward.h

@@ -25,6 +25,7 @@ class File;
 class IODevice;
 class LocalServer;
 class LocalSocket;
+class MappedFile;
 class MimeData;
 class NetworkJob;
 class NetworkResponse;