libpq-oauth: Fix Makefile dependencies

libpq-oauth: Fix Makefile dependencies

As part of 6225403f2, I'd removed the override for the `stlib` target,
since NAME no longer contains a major version number. But I forgot that
its dependencies are declared before Makefile.shlib is included; those
dependencies were then omitted entirely.

Per buildfarm member indri, which appears to be the only system so far
that's bothered by an empty archive.