LibWeb/CSP: Implement the frame-src directive

Author: https://github.com/Lubrsi
Commit: 1689353beb
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5276
Reviewed-by: https://github.com/shannonbooth ✅

Libraries/LibWeb/Forward.h

@@ -133,6 +133,7 @@ namespace Web::ContentSecurityPolicy::Directives {
 class ConnectSourceDirective;
 class Directive;
 class FontSourceDirective;
+class FrameSourceDirective;
 struct SerializedDirective;
 
 }