restoring jdk.internal.javac.PreviewFeature.Feature.SEALED_CLASSES, it is needed by the build

Author: vicente-romero-oracle

src/java.base/share/classes/jdk/internal/javac/PreviewFeature.java

@@ -54,6 +54,10 @@
     public boolean reflective() default false;
 
     public enum Feature {
+        /*
+         * This one can only be removed after JDK 17
+         */
+        SEALED_CLASSES,
         /**
          * A key for testing.
          */