Add type generics to SeekableIterator

Author: yottalogical

SPL/SPL.php

@@ -637,6 +637,9 @@ public function getChildren() {}
 /**
  * The Seekable iterator.
  * @link https://php.net/manual/en/class.seekableiterator.php
+ * @template TKey
+ * @template-covariant TValue
+ * @template-extends Iterator<TKey, TValue>
  */
 interface SeekableIterator extends Iterator
 {