gcr-certificate: Add (virtual) annotation

By adding (virtual) to a method, you tell the GObject Introspection scanner that it is the public API of a certain vfunc. Since you can't annotate a vfunc, GIR has a workaround where it will take over the annotations of the invoker.

In other words, to make sure the annotations of the return value of GcrCertificate.get_der_data() are understoor correctly, we just need to add the necessary tags to gcr_certificate_get_der_data() and mark it as (virtual get_der_data).

Fixes #37 (closed)