xtimer/xtimer.c: fix timeout is volatile
xtimer/xtimer.c: fix timeout is volatile

timeout is modified from interupt context so must be volatile

miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,OTAkeys/RIOT,basilfx/RIOT,cladmi/RIOT,yogo1212/RIOT,jasonatran/RIOT,authmillenon/RIOT,OlegHahm/RIOT,ant9000/RIOT,aeneby/RIOT,josephnoir/RIOT,josephnoir/RIOT,jasonatran/RIOT,ant9000/RIOT,aeneby/RIOT,yogo1212/RIOT,OlegHahm/RIOT,authmillenon/RIOT,basilfx/RIOT,rfuentess/RIOT,kaspar030/RIOT,jasonatran/RIOT,yogo1212/RIOT,OTAkeys/RIOT,jasonatran/RIOT,miri64/RIOT,miri64/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,mtausig/RIOT,yogo1212/RIOT,smlng/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,smlng/RIOT,rfuentess/RIOT,mtausig/RIOT,aeneby/RIOT,x3ro/RIOT,aeneby/RIOT,josephnoir/RIOT,miri64/RIOT,x3ro/RIOT,cladmi/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,cladmi/RIOT,kaspar030/RIOT,kaspar030/RIOT,authmillenon/RIOT,x3ro/RIOT,cladmi/RIOT,toonst/RIOT,kaspar030/RIOT,smlng/RIOT,yogo1212/RIOT,smlng/RIOT,OTAkeys/RIOT,authmillenon/RIOT,basilfx/RIOT,rfuentess/RIOT,toonst/RIOT,jasonatran/RIOT,josephnoir/RIOT,ant9000/RIOT,cladmi/RIOT,ant9000/RIOT,miri64/RIOT,smlng/RIOT,x3ro/RIOT,OTAkeys/RIOT,toonst/RIOT,toonst/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kaspar030/RIOT,x3ro/RIOT,mtausig/RIOT,kYc0o/RIOT,yogo1212/RIOT,OlegHahm/RIOT,rfuentess/RIOT,basilfx/RIOT,authmillenon/RIOT,mtausig/RIOT,basilfx/RIOT,aeneby/RIOT,toonst/RIOT,mtausig/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,rfuentess/RIOT