python-six: add host variant

While currently there is no in-tree Buildroot package which depends on
host-python-six, it can be needed to build external packages.

Signed-off-by: Julien Floret <julien.floret@6wind.com>
Tested-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>