fixed PH8.5 deprecation warning

Author: shimooka

lib/mcrypt.php

@@ -793,7 +793,11 @@ function phpseclib_mcrypt_generic_deinit(Base &$td)
     {
         $reflectionObject = new \ReflectionObject($td);
         $reflectionProperty = $reflectionObject->getProperty('key');
-        $reflectionProperty->setAccessible(true); // can be dropped in PHP 8.1.0+
+
+        if (PHP_VERSION_ID < 80100) {
+            $reflectionProperty->setAccessible(true);
+        }
+
         if (!strlen($reflectionProperty->getValue($td))) {
             trigger_error('mcrypt_generic_deinit(): Could not terminate encryption specifier', E_USER_WARNING);
             return false;