[small patch] Change datatype of ParallelMessagePending from "volatile bool" to "volatile sig_atomic_t"

Hi hackers,

While reviewing [1], I and Amit noticed that a flag ParallelMessagePending is defined
as "volatile bool", but other flags set by signal handlers are defined as "volatile sig_atomic_t".

The datatype has been defined in standard C,
and it says that variables referred by signal handlers should be "volatile sig_atomic_t".
(Please see my observation [2])

This may be not needed because any failures had been reported,
but I thought their datatype should be same and attached a small patch.

How do you think?

[1]: https://commitfest.postgresql.org/39/3621/
[2]:
https://www.postgresql.org/message-id/TYAPR01MB5866C056BB9F81A42B85D20BF54E9%40TYAPR01MB5866.jpnprd01.prod.outlook.com

Best Regards,
Hayato Kuroda
FUJITSU LIMITED