Suppress -Wunused-result warning about write().
pg_test_fsync's signal_cleanup() intentionally ignores the write()
result since there's not much we could do about it, but certain
compilers make that harder than it ought to be.
This was missed in commit
52e98d4502.
Reviewed-by: Tristan Partin, Peter Eisentraut
Discussion: https://postgr.es/m/
20231206161839.GA2828158%40nathanxps13