LibWeb/CSP: Implement the media-src directive

Author: https://github.com/Lubrsi
Commit: 1b12aa4d8e
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5308
Reviewed-by: https://github.com/shannonbooth

Libraries/LibWeb/Forward.h

@@ -136,6 +136,7 @@ class FontSourceDirective;
 class FrameSourceDirective;
 class ImageSourceDirective;
 class ManifestSourceDirective;
+class MediaSourceDirective;
 struct SerializedDirective;
 
 }