sys/xtimer: add missing msg.h include
sys/xtimer: add missing msg.h include

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