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/fw/forwarder.hpp b/daemon/fw/forwarder.hpp
index 3c80419..b561d6f 100644
--- a/daemon/fw/forwarder.hpp
+++ b/daemon/fw/forwarder.hpp
@@ -26,7 +26,7 @@
 #ifndef NFD_DAEMON_FW_FORWARDER_HPP
 #define NFD_DAEMON_FW_FORWARDER_HPP
 
-#include "common.hpp"
+#include "core/common.hpp"
 #include "core/scheduler.hpp"
 #include "forwarder-counters.hpp"
 #include "face-table.hpp"