LibCore: Add forward declaration for Core::Stream::File

Author: https://github.com/krkk
Commit: 4f699d0f58
Pull-request: https://github.com/SerenityOS/serenity/pull/16548
Reviewed-by: https://github.com/AtkinsSJ ✅
Reviewed-by: https://github.com/LucasChollet

Userland/Libraries/LibCore/Forward.h

@@ -38,6 +38,7 @@ class UDPServer;
 enum class TimerShouldFireWhenNotVisible;
 
 namespace Stream {
+class File;
 class Socket;
 class Stream;
 class BufferedSocketBase;