Make header self-contained

Add necessary include files for things used in the header.  (signal.h
needed for sig_atomic_t.)

src/include/replication/worker_internal.h

@@ -12,6 +12,8 @@
 #ifndef WORKER_INTERNAL_H
 #define WORKER_INTERNAL_H
 
+#include <signal.h>
+
 #include "access/xlogdefs.h"
 #include "catalog/pg_subscription.h"
 #include "datatype/timestamp.h"