mac802154: cfg: add missing include

Running make C=2 occurs warning:

symbol 'mac802154_config_ops' was not declared. Should it be static?

This patch adds a missing include in cfg.c to solve this warning.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reported-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>