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/tools/nfd-status/main.cpp b/tools/nfd-status/main.cpp
index a3033a6..b43e4cc 100644
--- a/tools/nfd-status/main.cpp
+++ b/tools/nfd-status/main.cpp
@@ -23,7 +23,7 @@
  * NFD, e.g., in COPYING.md file.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#include "version.hpp"
+#include "core/version.hpp"
 #include <ndn-cxx/security/validator-null.hpp>
 
 #include <boost/program_options/options_description.hpp>