ieee802154: sysfs: add missing include

Running make C=2 occurs in warnings:

symbol 'wpan_phy_class' was not declared. Should it be static?
symbol 'wpan_phy_sysfs_init' was not declared. Should it be static?
symbol 'wpan_phy_sysfs_exit' wasnot declared. Should it be static?

This patch adds a missing include "sysfs.h" to solve these warnings.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reported-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/ieee802154/sysfs.c

@@ -18,6 +18,7 @@
 #include <net/cfg802154.h>
 
 #include "core.h"
+#include "sysfs.h"
 
 static inline struct cfg802154_registered_device *
 dev_to_rdev(struct device *dev)