libtracker-miners-common: use syscall (SYS_tgkill) instead of tgkill

It's a bit less convenient, but more portable code. It has already lead to downstream patches in both alpine/postmarketOS and Open Embedded, since musl do not implement wrappers like tgkill (they are not part of any specification).

I hope I did not make any error in my understanding of this situation, and this can go in without many issues. But let me know if you would prefer some other solution to deal with this.