Merge bitcoin/bitcoin#27144: kernel: add missing include

49d01f32c9 kernel: add missing include (Cory Fields)

Pull request description:

  This syncs the cs_main definition/declaration.

  Noticed when experimenting with the external visibility of `cs_main`.

  Specifically, this is needed for the following to work as intended:
  ```c++
  __attribute__ ((visibility ("default"))) extern RecursiveMutex cs_main;
  ```

ACKs for top commit:
  fanquake:
    ACK 49d01f32c9

Tree-SHA512: ea0dbcf81959566f949d76c7dcd1e33de53e613519500c863bfb0ac8209665b1c12cff2daa7890d03b76debc4d046339ee7b3231adb71b128e9d5a8fa3132b6c

src/kernel/cs_main.cpp

@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <kernel/cs_main.h>
 #include <sync.h>
 
 RecursiveMutex cs_main;