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