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/table/name-tree-entry.hpp b/daemon/table/name-tree-entry.hpp
index 23acdd0..4d7ad0c 100644
--- a/daemon/table/name-tree-entry.hpp
+++ b/daemon/table/name-tree-entry.hpp
@@ -26,7 +26,7 @@
 #ifndef NFD_DAEMON_TABLE_NAME_TREE_ENTRY_HPP
 #define NFD_DAEMON_TABLE_NAME_TREE_ENTRY_HPP
 
-#include "common.hpp"
+#include "core/common.hpp"
 #include "table/fib-entry.hpp"
 #include "table/pit-entry.hpp"
 #include "table/measurements-entry.hpp"