oauth: Correct test dependency on oauth_hook_client

oauth: Correct test dependency on oauth_hook_client

The oauth_validator tests missed the lessons of c89525d57 et al, so
certain combinations of command-line build order and `meson test`
options can result in

    Command 'oauth_hook_client' not found in [...] at src/test/perl/PostgreSQL/Test/Utils.pm line 427.

Add the missing dependency on the test executable. This fixes, for
example,

    $ ninja clean && ninja meson-test-prereq && PG_TEST_EXTRA=oauth meson test --no-rebuild

Reported-by: Jonathan Gonzalez V. <jonathan.abdiel@gmail.com>
Author: Jonathan Gonzalez V. <jonathan.abdiel@gmail.com>
Discussion: https://postgr.es/m/6e8f4f7c23faf77c4b6564c4b7dc5d3de64aa491.camel@gmail.com
Discussion: https://postgr.es/m/qh4c5tvkgjef7jikjig56rclbcdrrotngnwpycukd2n3k25zi2%4044hxxvtwmgum
Backpatch-through: 18