core: slim down `common.hpp`

Change-Id: I875c35147edd2261fbaa24e809c170d5cd9b94d3
diff --git a/daemon/face/channel.hpp b/daemon/face/channel.hpp
index 19fcd35..3ecb986 100644
--- a/daemon/face/channel.hpp
+++ b/daemon/face/channel.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2022,  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,
@@ -29,6 +29,8 @@
 #include "channel-log.hpp"
 #include "face-common.hpp"
 
+#include <functional>
+
 namespace nfd::face {
 
 /** \brief Represents a channel that listens on a local endpoint.