Fix typo in reference to __FreeBSD__.
Commit
a2a8acd152 introduced a platform-dependent mechanism to prevent
developers from referencing errno in the argument list of
elog()/ereport(), but didn't use the right macro to detect FreeBSD, so
it didn't actually work there.
Reported-by: Japin Li <japinli@hotmail.com>
Discussion: https://postgr.es/m/MEYP282MB16693AAEEF84F47D8F7CA007B6E69%40MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM