Mark ParallelMessagePending as sig_atomic_t

ParallelMessagePending was previously marked as a boolean which should
be fine on modern platforms, but the C standard recommends the use of
sig_atomic_t for variables manipulated in signal handlers.

Author: Hayato Kuroda
Discussion: https://postgr.es/m/TYAPR01MB58667C15A95A234720F4F876F5529@TYAPR01MB5866.jpnprd01.prod.outlook.com

src/backend/access/transam/parallel.c

@@ -113,7 +113,7 @@ typedef struct FixedParallelState
 int			ParallelWorkerNumber = -1;
 
 /* Is there a parallel message pending which we need to receive? */
-volatile bool ParallelMessagePending = false;
+volatile sig_atomic_t ParallelMessagePending = false;
 
 /* Are we initializing a parallel worker? */
 bool		InitializingParallelWorker = false;

src/include/access/parallel.h

@@ -54,7 +54,7 @@ typedef struct ParallelWorkerContext
 	shm_toc    *toc;
 } ParallelWorkerContext;
 
-extern PGDLLIMPORT volatile bool ParallelMessagePending;
+extern PGDLLIMPORT volatile sig_atomic_t ParallelMessagePending;
 extern PGDLLIMPORT int ParallelWorkerNumber;
 extern PGDLLIMPORT bool InitializingParallelWorker;