core: move common.hpp to core/

Headers in core/ are now included as `#include "core/*.hpp"`.
This allows NFD to rely less on include paths when used as a submodule in
another codebase, such as ndnSIM and NFD-Android.

refs #3127

Change-Id: Ia38f3c97427e17ff7b5401281f1fe875b5d5313e
diff --git a/daemon/nfd.hpp b/daemon/nfd.hpp
index dd97cf4..d8fd12e 100644
--- a/daemon/nfd.hpp
+++ b/daemon/nfd.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2014-2015,  Regents of the University of California,
+ * Copyright (c) 2014-2016,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -26,7 +26,6 @@
 #ifndef NFD_DAEMON_NFD_HPP
 #define NFD_DAEMON_NFD_HPP
 
-#include "common.hpp"
 #include "core/config-file.hpp"
 #include "core/scheduler.hpp"