Restore the default value of rhev_build in the spec file

Change-Id: I8deca956e80bcacad1e6446975fe916d9247abdd
Signed-off-by: Martin Sivak <mars@montik.net>
Reviewed-on: http://gerrit.ovirt.org/35794
Tested-by: Martin Sivák <msivak@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>