common.hpp: avoid conflict between std::placeholders and Boost placeholders

refs #2109

Change-Id: Ia231500e6adabd10179db4aeeaf799a28fee07dc
diff --git a/src/security/conf/common.hpp b/src/security/conf/common.hpp
index a84789c..3c79b7b 100644
--- a/src/security/conf/common.hpp
+++ b/src/security/conf/common.hpp
@@ -24,6 +24,7 @@
 #ifndef NDN_SECURITY_CONF_COMMON_HPP
 #define NDN_SECURITY_CONF_COMMON_HPP
 
+#include "../../common.hpp"
 #include <string>
 #include <boost/property_tree/ptree.hpp>