core: slim down `common.hpp`

Change-Id: I875c35147edd2261fbaa24e809c170d5cd9b94d3
diff --git a/tests/daemon/global-io-fixture.hpp b/tests/daemon/global-io-fixture.hpp
index 5422e72..a18a587 100644
--- a/tests/daemon/global-io-fixture.hpp
+++ b/tests/daemon/global-io-fixture.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,
@@ -28,6 +28,10 @@
 
 #include "tests/clock-fixture.hpp"
 
+namespace boost::asio {
+class io_context;
+} // namespace boost::asio
+
 namespace nfd::tests {
 
 /**