face: Limit exposure of EthernetFace implementation

It seems that gcc 4.6 on Ubuntu 12.04 has some issue with
std::unordered_map in EthernetFace implementation, causing it segfault
in some cases.

Change-Id: I0c5a3be66e357e2bd83841d833d601c2fba6936a
Refs: #2347
diff --git a/daemon/mgmt/face-manager.cpp b/daemon/mgmt/face-manager.cpp
index f5d849b..768f1b9 100644
--- a/daemon/mgmt/face-manager.cpp
+++ b/daemon/mgmt/face-manager.cpp
@@ -38,6 +38,7 @@
 
 #ifdef HAVE_LIBPCAP
 #include "face/ethernet-factory.hpp"
+#include "face/ethernet-face.hpp"
 #endif // HAVE_LIBPCAP
 
 #ifdef HAVE_WEBSOCKET