LibWeb/CSP: Implement the img-src directive

Author: https://github.com/Lubrsi
Commit: 715061fb79
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5276
Reviewed-by: https://github.com/shannonbooth ✅

Libraries/LibWeb/Forward.h

@@ -134,6 +134,7 @@ class ConnectSourceDirective;
 class Directive;
 class FontSourceDirective;
 class FrameSourceDirective;
+class ImageSourceDirective;
 struct SerializedDirective;
 
 }