build: move `config.hpp` private header to `detail` subdir

Change-Id: I9f3bb2e288450f002ecec77b52411647ed4fea5a
diff --git a/src/common.hpp b/src/common.hpp
index 4232aa0..629463b 100644
--- a/src/common.hpp
+++ b/src/common.hpp
@@ -20,7 +20,7 @@
 #ifndef NDN_NAC_COMMON_HPP
 #define NDN_NAC_COMMON_HPP
 
-#include "config.hpp"
+#include "detail/config.hpp"
 
 #ifdef NAC_WITH_TESTS
 #define NAC_VIRTUAL_WITH_TESTS virtual