net/gnrc_sock: do not include xtimer.h without xtimer module
net/gnrc_sock: do not include xtimer.h without xtimer module

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