spec: add dmidecode as prereq

spec: add dmidecode as prereq

https://bugzilla.redhat.com/show_bug.cgi?id=754909 complains that
because libvirt didn't require dmidecode, that the logs are noisy
and virConnectGetSysinfo needlessly fails.  Even 'virt-what' requires
dmidecode, so it's not that onerous of a dependency.  We may be
able to drop this in the future when we move to parsing sysfs data,
but for now, listing the dependency will help matters.

* libvirt.spec.in (Requires): Sort Requires before BuildRequires.
Add dmidecode.