net-sysfs: fix missing <linux/of_net.h>

The of_find_net_device_by_node() function is defined in
<linux/of_net.h> but not included in the .c file that
implements it. Fix the following warning by including the
header:

net/core/net-sysfs.c:1494:19: warning: symbol 'of_find_net_device_by_node' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>