sys/net/gnrc_lwmac: add missing xtimer dependency
sys/net/gnrc_lwmac: add missing xtimer dependency

kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT