rpm_spec: Add ethtool as a requirement

The new ipwrapper link type detection depends on the ethtool binary
that is commonly installed by default but not on minimal
installations. This patch ensures that it is not missing.

Change-Id: Idc8c806decad74b3e90d485eb8e9a57eeac41ebf
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22177
Reviewed-by: Dan Kenigsberg <danken@redhat.com>