Fix replication/worker_internal.h to compile without other headers.

This header hasn't changed recently, so the fact that it now fails
headerscheck/cpluspluscheck testing must be due to changes in what
it includes.  Probably f2191679 is to blame, but I didn't try to
verify that.

Discussion: https://postgr.es/m/3699703.1595016554@sss.pgh.pa.us

src/include/replication/worker_internal.h

@@ -18,6 +18,8 @@
 #include "catalog/pg_subscription.h"
 #include "datatype/timestamp.h"
 #include "storage/lock.h"
+#include "storage/spin.h"
+
 
 typedef struct LogicalRepWorker
 {