LibWeb: Add LinearGradientStyleValue

Author: https://github.com/MacDue
Commit: 259bb30c35
Pull-request: https://github.com/SerenityOS/serenity/pull/14564
Reviewed-by: https://github.com/AtkinsSJ

Userland/Libraries/LibWeb/Forward.h

@@ -62,6 +62,7 @@ class InitialStyleValue;
 class Length;
 class LengthPercentage;
 class LengthStyleValue;
+class LinearGradientStyleValue;
 class ListStyleStyleValue;
 class MediaList;
 class MediaQuery;