kni: fix build with kernel < 3.3 with netdev_features_t backport

The netdev_features_t typedef appeared in Linux 3.3, but checking the kernel
version isn't enough with some distributions (such as Debian Wheezy) that
backported it into 3.2, causing a compilation failure due to redefinition.

Since the presence of a typedef can't be tested at compile time, this commit
adds type kni_netdev_features_t, which, depending on the kernel version,
translates either to u32 or netdev_features_t.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>