agent: Disable GDM plugin building on Fedora 19+

Fedora 19 does no longer have a GDM with plugin capabilities.
To allow proper building of the package gdm plugins are no longer
built for Fedora 19 and higher.

Change-Id: Iff3f5a3c7c12d9c6ef198aefe163d9633d8364b3
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>