postgresqlPackages.pg_anonymizer: fix meta

Previously the entire `meta` section from `pg-dump-anon` was copied
over including `mainProgram` which doesn't belong here. To avoid similar
issues, fields from the meta section of pg-dump-anon are copied over
explicitly.

pkgs/servers/sql/postgresql/ext/anonymizer.nix

···
     EOF
   '';
 
-  meta = pg-dump-anon.meta // {
+  meta = lib.getAttrs [ "homepage" "maintainers" "license" ] pg-dump-anon.meta // {
     description = "Extension to mask or replace personally identifiable information (PII) or commercially sensitive data from a PostgreSQL database";
   };
 })