fix clang 3.3 warning in linux kore_platform_init

```
src/linux.c:55:50: warning: comparison of constant -1 with expression of type
      'u_int16_t' (aka 'unsigned short') is always false
      [-Wtautological-constant-out-of-range-compare]
        if ((cpu_count = sysconf(_SC_NPROCESSORS_ONLN)) == -1) {
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~
```