refactor: add kernel/cs_main.*

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

src/validationinterface.h

@@ -6,13 +6,13 @@
 #ifndef BITCOIN_VALIDATIONINTERFACE_H
 #define BITCOIN_VALIDATIONINTERFACE_H
 
+#include <kernel/cs_main.h>
 #include <primitives/transaction.h> // CTransaction(Ref)
 #include <sync.h>
 
 #include <functional>
 #include <memory>
 
-extern RecursiveMutex cs_main;
 class BlockValidationState;
 class CBlock;
 class CBlockIndex;