kconfig: add xstrdup() helper

We already have xmalloc(), xcalloc(), and xrealloc(().  Add xstrdup()
as well to save tedious error handling.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/zconf.y

@@ -127,7 +127,7 @@ no_mainmenu_stmt: /* empty */
 	 * later regardless of whether it comes from the 'prompt' in
 	 * mainmenu_stmt or here
 	 */
-	menu_add_prompt(P_MENU, strdup("Linux Kernel Configuration"), NULL);
+	menu_add_prompt(P_MENU, xstrdup("Linux Kernel Configuration"), NULL);
 };