sys/timex: change type of NS_PER_SEC
sys/timex: change type of NS_PER_SEC

Having NS_PER_SEC defined as `1000000000U` is quite a foot gun, as
multiplication even with small numbers in 32 bit quickly overflows. Using the
`1000000000LLU` instead forces 64 bit math.

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