face: mark UnixStreamFace constructor explicit.

The keyword was accidentally removed in commit
355c066fe188ca0a1b51a43a39f4e9ef402d1448.

Change-Id: I1e345cf5ccb1d5ddc56419fb19493eb446590880
diff --git a/daemon/face/unix-stream-face.hpp b/daemon/face/unix-stream-face.hpp
index 02ccbb5..8492f54 100644
--- a/daemon/face/unix-stream-face.hpp
+++ b/daemon/face/unix-stream-face.hpp
@@ -25,6 +25,7 @@
 public:
   typedef boost::asio::local::stream_protocol protocol;
 
+  explicit
   UnixStreamFace(const shared_ptr<protocol::socket>& socket);
 };