conf: schemas: Include 'privatedata.rng' in installed schema files

conf: schemas: Include 'privatedata.rng' in installed schema files

The privatedata.rng file was accidentally left uninstalled, but it's
referenced by other schema files effectively breaking validation of XMLs
in new installations.

Change to libvirt.spec is not needed as we include all installed schemas
via a wildcard.

Fixes: d8ceacdc87907a3c8656f7ee815ed32f06fe5c7f
Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>