testing/rtkit: new aport

https://github.com/heftig/rtkit D-Bus system service that enables real time scheduling on request

The syscall sched_getscheduler and sched_setscheduler have previously been implemented in musl, but now dropped in favor or better POSIX compliance. Still, this package needs them to work. A patch was added to vendor the old implementations in - each syscall is about 1 line of code.