Unmark gen_random_uuid() function leakproof

Hi,

While reviewing UUIDv7 patch[1], I realized gen_random_uuid() is
marked leakproof even though it doesn't take arguments. The functions
without arguments don't need to be marked leakproof in principle. This
is the sole function that has no arguments and is listed in the "List
of built-in leakproof functions" in opr_sanity.sql. I've attached the
patch for fixing it and for better consistency with new UUID
generation functions discussed on that thread.

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com