Fix define used for COREML_ALLOW_FALLBACK

[jcsoo]

CMakeLists.txt defines WHISPER_USE_COREML_ALLOW_FALLBACK when COREML_ALLOW_FALLBACK=1 is used:

        if (WHISPER_COREML_ALLOW_FALLBACK)
            set(WHISPER_EXTRA_FLAGS ${WHISPER_EXTRA_FLAGS} -DWHISPER_USE_COREML_ALLOW_FALLBACK)
        endif()

But whisper.cpp actually uses WHISPER_COREML_ALLOW_FALLBACK:

#ifndef WHISPER_COREML_ALLOW_FALLBACK
        return nullptr;
#endif