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

net/mac802154/cfg.c

@@ -16,6 +16,7 @@
 #include <net/cfg802154.h>
 
 #include "ieee802154_i.h"
+#include "cfg.h"
 
 static struct net_device *
 ieee802154_add_iface_deprecated(struct wpan_phy *wpan_phy,