[build] Add netgroup.cpp|h

These aren't used yet.

src/Makefile.am

@@ -180,6 +180,7 @@ BITCOIN_CORE_H = \
   net_types.h \
   netaddress.h \
   netbase.h \
+  netgroup.h \
   netmessagemaker.h \
   node/blockstorage.h \
   node/caches.h \
@@ -352,6 +353,7 @@ libbitcoin_node_a_SOURCES = \
   init.cpp \
   mapport.cpp \
   net.cpp \
+  netgroup.cpp \
   net_processing.cpp \
   node/blockstorage.cpp \
   node/caches.cpp \

src/netgroup.cpp

@@ -0,0 +1,5 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <netgroup.h>

src/netgroup.h

@@ -0,0 +1,13 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_NETGROUP_H
+#define BITCOIN_NETGROUP_H
+
+/**
+ * Netgroup manager
+ */
+class NetGroupManager {};
+
+#endif // BITCOIN_NETGROUP_H