face: move Channel and subclasses into nfd::face namespace

Change-Id: I163ef20e11b65d19d68585ec075fd3cd224a573b
diff --git a/daemon/face/udp-channel.hpp b/daemon/face/udp-channel.hpp
index e86e8e9..b9b71a8 100644
--- a/daemon/face/udp-channel.hpp
+++ b/daemon/face/udp-channel.hpp
@@ -30,6 +30,7 @@
 #include "udp-protocol.hpp"
 
 namespace nfd {
+namespace face {
 
 /**
  * \brief Class implementing UDP-based channel to create faces
@@ -133,6 +134,7 @@
   return m_socket.is_open();
 }
 
+} // namespace face
 } // namespace nfd
 
 #endif // NFD_DAEMON_FACE_UDP_CHANNEL_HPP