Reduce inclusions of <boost/asio.hpp> in public headers

And especially, stop including it from the precompiled header
"common-pch.hpp". This is a necessary step in order to avoid
conflicts between net/if.h (pulled in by Asio) and linux/if.h
(required by future commits).

Change-Id: I0207be0d2be796d8ef369062a8d5195fedfef6b0
diff --git a/tests/unit-tests/util/dns.t.cpp b/tests/unit-tests/util/dns.t.cpp
index 630295b..be94ba9 100644
--- a/tests/unit-tests/util/dns.t.cpp
+++ b/tests/unit-tests/util/dns.t.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2015 Regents of the University of California.
+ * Copyright (c) 2013-2016 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -23,6 +23,8 @@
 
 #include "boost-test.hpp"
 #include "../network-configuration-detector.hpp"
+
+#include <boost/asio.hpp>
 #include <boost/lexical_cast.hpp>
 
 namespace ndn {