[OAuth] Missing dependency on oauth_validator test

Hi,

While working in another patch[1] I found that the test for the
oauth_validator was failing when running the test using:

meson test -C build --setup --suite oauth_validator

This was due to the binary `oauth_hook_client` being missing because I
build the code without enabling the OAuth test using `PG_EXTRA_TEST`.

I'm attaching a small patch that add the binary dependency in the
meson.build file, thus, it will be build before running the test.


Regards,

[1] https://commitfest.postgresql.org/patch/6183/

--
Jonathan Gonzalez V. <jonathan.abdiel@gmail.com>
EnterpriseDB