libpq: Authorize pthread_exit() in libpq_check

libpq: Authorize pthread_exit() in libpq_check

pthread_exit() is added to the list of symbols allowed when building
libpq.  This has been reported as possible when libpq is statically
linked to libcrypto, where pthread_exit() could be called.

Reported-by: Torsten Rupp <[email protected]>
Author: Nazir Bilal Yavuz <[email protected]>
Reviewed-by: Daniel Gustafsson <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Discussion: https://postgr.es/m/19095-6d8256d0c37d4be2@postgresql.org