nvmetcli: add python-six to rpm package building

From experience, if python-six isn't installed on a Fedora
system, hard-to-decipher errors can occur trying to use
nvmetcli.

Signed-off-by: Jay Freyensee <james.p.freyensee@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>