Define default OPTION_TABS_ON_SPECIALS option

If the option is not default, system will throw OutOfBoundsException
when trying to access OPTION_TABS_ON_SPECIALS value.

Bug: T225279
Change-Id: I01ff016caea0bfd961c71e15388d670cd93acc7f

includes/SkinOptions.php

@@ -59,6 +59,8 @@ final class SkinOptions {
 		self::OPTIONS_TALK_AT_TOP => false,
 		self::OPTIONS_HISTORY_PAGE_ACTIONS => false,
 		self::OPTION_OVERFLOW_SUBMENU => false,
+		/** Whether to show tabs on special pages */
+		self::OPTION_TABS_ON_SPECIALS => false,
 	];
 
 	/**