[build] Add netgroup.cpp|h

These aren't used yet.

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>