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/face/ethernet-factory.hpp b/daemon/face/ethernet-factory.hpp
index cc8516a..ed81f5f 100644
--- a/daemon/face/ethernet-factory.hpp
+++ b/daemon/face/ethernet-factory.hpp
@@ -26,11 +26,13 @@
 #ifndef NFD_DAEMON_FACE_ETHERNET_FACTORY_HPP
 #define NFD_DAEMON_FACE_ETHERNET_FACTORY_HPP
 
-#include "ethernet-face.hpp"
 #include "protocol-factory.hpp"
+#include "core/network-interface.hpp"
 
 namespace nfd {
 
+class EthernetFace;
+
 class EthernetFactory : public ProtocolFactory
 {
 public: