Fix libosv build with python 2

Commit c00e6e14bc0164b5bb38b723ac1499fa4b9028b9 attempted to fix libosv.py
to also support python 2, but it used a "__future__" import statement
which was not necessary, and doesn't work (Python 2.7.5 says it is misplaced
and aborts).

Removing this statement makes build work again on Fedora 20 with its
Python 2.7.5.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>