pgsql: Update comment related to C99

Update comment related to C99

One could do more work here to eliminate the Windows difference
described in the comment, but that can be a separate project.  The
purpose of this change is to update comments that might confusingly
indicate that C99 is not required.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/170308e6-a7a3-4484-87b2-f960bb564afa%40eisentraut.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/376c649634bb3fbfad79940f996e2fcb4c2ea6bf

Modified Files
--------------
src/bin/pg_dump/dumputils.h | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)