Busted includes somewhere near worker_internal.h

headerscheck and cpluspluscheck are both unhappy about this:

./src/include/replication/worker_internal.h:49:2: error: unknown type name 'slock_t'
  slock_t  relmutex;
  ^~~~~~~

Now, worker_internal.h itself hasn't changed in some time.
I conclude that somebody rearranged one of the header files
it depends on.  Anyone have an idea what the relevant change
was?  Should we just include spin.h here, or is there a
better fix?

            regards, tom lane