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

Change-Id: I163ef20e11b65d19d68585ec075fd3cd224a573b
diff --git a/daemon/face/channel.hpp b/daemon/face/channel.hpp
index 1c0452d..300435c 100644
--- a/daemon/face/channel.hpp
+++ b/daemon/face/channel.hpp
@@ -29,6 +29,7 @@
 #include "face.hpp"
 
 namespace nfd {
+namespace face {
 
 /**
  * \brief Prototype for the callback that is invoked when the face
@@ -82,6 +83,7 @@
 void
 connectFaceClosedSignal(Face& face, const std::function<void()>& f);
 
+} // namespace face
 } // namespace nfd
 
 #endif // NFD_DAEMON_FACE_CHANNEL_HPP