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/face/generic-link-service.hpp b/daemon/face/generic-link-service.hpp
index dde6990..c9adcd7 100644
--- a/daemon/face/generic-link-service.hpp
+++ b/daemon/face/generic-link-service.hpp
@@ -26,7 +26,7 @@
 #ifndef NFD_DAEMON_FACE_GENERIC_LINK_SERVICE_HPP
 #define NFD_DAEMON_FACE_GENERIC_LINK_SERVICE_HPP
 
-#include "common.hpp"
+#include "core/common.hpp"
 #include "core/logger.hpp"
 
 #include "link-service.hpp"