vdsm.spec: BuildRequires: python-pthreading

Moving python-pthreading to be a generic BuildRequirement

Change-Id: If996c366cac42e149193651d2d8a6dbde6d8c81e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=891542
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10840
Reviewed-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>