pgsql: Suppress -Wunused-result warning about write().

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c2a465b2c94fb44211c350f73b5a11978d3b4536

Modified Files
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src/bin/pg_test_fsync/pg_test_fsync.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)