EthernetFace: report packets dropped by the kernel if debug is enabled

Change-Id: I8b0f9774c4ad785da476aba88f0b6a212256bf51
Refs: #2441
diff --git a/daemon/face/ethernet-face.hpp b/daemon/face/ethernet-face.hpp
index 7a3d881..cb6303b 100644
--- a/daemon/face/ethernet-face.hpp
+++ b/daemon/face/ethernet-face.hpp
@@ -162,6 +162,11 @@
   unique_ptr<ndnlp::Slicer> m_slicer;
   std::unordered_map<ethernet::Address, Reassembler> m_reassemblers;
   static const time::nanoseconds REASSEMBLER_LIFETIME;
+
+#ifdef _DEBUG
+  /// number of packets dropped by the kernel, as reported by libpcap
+  unsigned int m_nDropped;
+#endif
 };
 
 } // namespace nfd