netdev-linux: Avoid depending on kernel definition of rtnl_link_stats64.

We have to define our own with some kernel headers, so we might as well do
it everywhere, especially since there seems to be a problem with detecting
the presence of the definition with at least some kernels.

Reported-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>