gluster: Fix for build failure when with_gluster=0

Currently vdsmapi-gluster-schema.json file is not collected
by anything when built with_gluster=0, which is causing build
failure. Modified spec to exclude vdsmapi-gluster-schema.json
file when gluster is not enabled.

Change-Id: I0025365e367e4f1b294ec6439df90b7ed6538fb5
Signed-off-by: ndarshan <dnarayan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33240
Reviewed-by: Piotr Kliczewski <piotr.kliczewski@gmail.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>