Update EnumHelper to be compatible with Eclipse's OpenJ9 JVM. (#5712)

src/main/java/net/minecraftforge/common/util/EnumHelper.java

@@ -250,6 +250,8 @@ public class EnumHelper
     {
         blankField(enumClass, "enumConstantDirectory");
         blankField(enumClass, "enumConstants");
+        //Open J9
+        blankField(enumClass, "enumVars");
     }
 
     @Nullable