refactor: add kernel/cs_main.*

Co-authored-by: Anthony Towns <aj@erisian.com.au>

src/kernel/cs_main.cpp

@@ -0,0 +1,7 @@
+// Copyright (c) 2023 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 <sync.h>
+
+RecursiveMutex cs_main;