LibWeb: Add ReadableStreamDefaultController

Author: https://github.com/mattco98
Commit: bc9919178e
Pull-request: https://github.com/SerenityOS/serenity/pull/18109
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/Forward.h

@@ -418,6 +418,7 @@ class Selection;
 
 namespace Web::Streams {
 class ReadableStream;
+class ReadableStreamDefaultController;
 class ReadableStreamDefaultReader;
 class ReadableStreamGenericReaderMixin;
 class ReadRequest;