drivers/mrf24j40: remove unneeded GNRC header inclusion
drivers/mrf24j40: remove unneeded GNRC header inclusion

RIOT-OS/RIOT,ant9000/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,authmillenon/RIOT,authmillenon/RIOT,ant9000/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,kYc0o/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,jasonatran/RIOT,authmillenon/RIOT,authmillenon/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,OTAkeys/RIOT,authmillenon/RIOT,jasonatran/RIOT,kYc0o/RIOT,kaspar030/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OlegHahm/RIOT