Core: Prefix LOG_LEVEL_ with LIBUSB_ to avoid conflicts

Core: Prefix LOG_LEVEL_ with LIBUSB_ to avoid conflicts

* The LOG_LEVEL_ enums, that were moved to the public API
  in 933a319469bcccc962031c989e39d9d1f44f2885 may conflict
  with applications/headers that also define their own
  LOG_LEVEL_ values internally.
* As a matter of fact, as per Trac #31, this produces a
  conflict with libusb-compat, as it defines its own levels.