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

net/core/net-sysfs.c

@@ -24,6 +24,7 @@
 #include <linux/jiffies.h>
 #include <linux/pm_runtime.h>
 #include <linux/of.h>
+#include <linux/of_net.h>
 
 #include "net-sysfs.h"