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>