core: slim down `common.hpp`

Change-Id: I875c35147edd2261fbaa24e809c170d5cd9b94d3
diff --git a/daemon/table/name-tree-iterator.hpp b/daemon/table/name-tree-iterator.hpp
index 1b97c92..e4e97ef 100644
--- a/daemon/table/name-tree-iterator.hpp
+++ b/daemon/table/name-tree-iterator.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2023,  Regents of the University of California,
+ * Copyright (c) 2014-2024,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -31,6 +31,8 @@
 #include <boost/operators.hpp>
 #include <boost/range/iterator_range_core.hpp>
 
+#include <functional>
+
 namespace nfd::name_tree {
 
 class NameTree;