util: NetworkMonitor: fix build on unsupported platforms

Change-Id: I08ef2c3858e0aea35ec2de739ddd86734b40358d
Refs: #3353
diff --git a/src/util/network-monitor.cpp b/src/util/network-monitor.cpp
index ccb5418..6689711 100644
--- a/src/util/network-monitor.cpp
+++ b/src/util/network-monitor.cpp
@@ -37,10 +37,22 @@
 class NetworkMonitor::Impl
 {
 public:
-  Impl(NetworkMonitor& nm, boost::asio::io_service& io)
+  Impl(NetworkMonitor&, boost::asio::io_service&)
   {
     BOOST_THROW_EXCEPTION(Error("Network monitoring is not supported on this platform"));
   }
+
+  shared_ptr<NetworkInterface>
+  getNetworkInterface(const std::string&) const
+  {
+    return {};
+  }
+
+  std::vector<shared_ptr<NetworkInterface>>
+  listNetworkInterfaces() const
+  {
+    return {};
+  }
 };
 
 } // namespace util