Apply correct number of workers based on cores for OSX

Fix for the following issue on a multi-core system.

```
[66412] src/worker.c:89 - kore_worker_init(): system has 0 cpu's
[66412] src/worker.c:90 - kore_worker_init(): starting 4 workers
[66412] src/worker.c:92 - kore_worker_init(): more workers then cpu's
```

It should detect the system to have proper number of cpus (cores) and then spawn workers based on that.

```bash
$ sysctl -n machdep.cpu.brand_string
Intel(R) Core(TM) i7-3667U CPU @ 2.00GHz
$ sysctl hw.availcpu
hw.availcpu = 4
$ sysctl hw.ncpu
hw.ncpu: 4
```