whisper : expose API to let user control log output (ggml-org#1060)

* expose api to let user control log output

Add
  whisper_set_log_callback()
that lets user set a callback for log messages.

Change all the
  fprintf(stderr, ...)
to call via the above.

* whisper : add <cstdarg>

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

Author: evmar