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/tests/daemon/face/dummy-transport.hpp b/tests/daemon/face/dummy-transport.hpp
index 4dfda76..951b1a5 100644
--- a/tests/daemon/face/dummy-transport.hpp
+++ b/tests/daemon/face/dummy-transport.hpp
@@ -26,7 +26,7 @@
 #ifndef NFD_TESTS_DAEMON_FACE_DUMMY_TRANSPORT_HPP
 #define NFD_TESTS_DAEMON_FACE_DUMMY_TRANSPORT_HPP
 
-#include "common.hpp"
+#include "core/common.hpp"
 
 #include "face/transport.hpp"