lib: changed var to const in linkedlist

PR-URL: #8609
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

lib/_linklist.js

@@ -17,7 +17,7 @@ exports.peek = peek;
 
 // remove the most idle item from the list
 function shift(list) {
-  var first = list._idlePrev;
+  const first = list._idlePrev;
   remove(first);
   return first;
 }