No more:
```
// Actually declared in validation.cpp; can't include because of circular dependency.
extern RecursiveMutex cs_main;
```
Note that including `validation.h` still implies that `cs_main` is available. In the long term we can expect `cs_main` to become a validation specific lock. No need to add the new include in all source files already including validation.
Co-authored-by: Anthony Towns <aj@erisian.com.au>
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/26521 | core#26521]]
Depends on D15790