Add missing entry to oauth_validator test .gitignore

Commit b3f0be7 accidentally missed adding the oauth client test
binary to the relevant .gitignore.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2839306.1740082041@sss.pgh.pa.us

Author: danielgustafsson

src/test/modules/oauth_validator/.gitignore

@@ -2,3 +2,4 @@
 /log/
 /results/
 /tmp_check/
+/oauth_hook_client