core: slim down `common.hpp`

Change-Id: I875c35147edd2261fbaa24e809c170d5cd9b94d3
diff --git a/daemon/main.cpp b/daemon/main.cpp
index 725b7b6..cc3c58a 100644
--- a/daemon/main.cpp
+++ b/daemon/main.cpp
@@ -291,9 +291,9 @@
   }
 
   const std::string boostBuildInfo =
-      "with Boost version " + to_string(BOOST_VERSION / 100000) +
-      "." + to_string(BOOST_VERSION / 100 % 1000) +
-      "." + to_string(BOOST_VERSION % 100);
+      "with Boost version " + std::to_string(BOOST_VERSION / 100000) +
+      "." + std::to_string(BOOST_VERSION / 100 % 1000) +
+      "." + std::to_string(BOOST_VERSION % 100);
   const std::string pcapBuildInfo =
 #ifdef NFD_HAVE_LIBPCAP
       "with " + std::string(pcap_lib_version());
@@ -302,9 +302,9 @@
 #endif
   const std::string wsBuildInfo =
 #ifdef NFD_HAVE_WEBSOCKET
-      "with WebSocket++ version " + to_string(websocketpp::major_version) +
-      "." + to_string(websocketpp::minor_version) +
-      "." + to_string(websocketpp::patch_version);
+      "with WebSocket++ version " + std::to_string(websocketpp::major_version) +
+      "." + std::to_string(websocketpp::minor_version) +
+      "." + std::to_string(websocketpp::patch_version);
 #else
       "without WebSocket++";
 #endif