Memory leak in volk_load_preferences

Looks like preferences are loaded on the first access, but never destroyed. Can't find any helped method to properly cleanup.

Got this while running valgrind:

```
==29410== 11,904 bytes in 1 blocks are still reachable in loss record 1 of 1
==29410==    at 0x4849F4C: realloc (vg_replace_malloc.c:785)
==29410==    by 0x497CD5F: volk_load_preferences (in /usr/local/lib/libvolk.so.2.0)
```