Fix some libretro-common usage

[RobLoach]

No description provided.

[konsumer]

I tested, and it seems to work, but builds libnull0_libretro.a, so I added SHARED and that results in this:

Undefined symbols for architecture x86_64:
  "_audio_mixer_destroy", referenced from:
      _pntr_load_sound_from_memory in main.c.o
      _pntr_unload_sound in main.c.o
      _null0_new_sfx in main.c.o
      _null0_unload_sound in main.c.o
      _wamr_null0_unload_sound in main.c.o
  "_audio_mixer_done", referenced from:
      _pntr_app_platform_close in main.c.o
      _retro_load_game in main.c.o
      _pntr_app_close in main.c.o
  "_audio_mixer_init", referenced from:
      _pntr_app_platform_init in main.c.o
  "_audio_mixer_load_ogg", referenced from:
      _pntr_load_sound_from_memory in main.c.o
  "_audio_mixer_load_wav", referenced from:
      _pntr_load_sound_from_memory in main.c.o
      _null0_new_sfx in main.c.o
  "_audio_mixer_mix", referenced from:
      _retro_audio_cb in main.c.o
  "_audio_mixer_play", referenced from:
      _pntr_play_sound in main.c.o
      _null0_play_sound in main.c.o
      _wamr_null0_play_sound in main.c.o
  "_audio_mixer_stop", referenced from:
      _pntr_unload_sound in main.c.o
      _pntr_stop_sound in main.c.o
      _null0_stop_sound in main.c.o
      _null0_unload_sound in main.c.o
      _wamr_null0_stop_sound in main.c.o
      _wamr_null0_unload_sound in main.c.o
  "_convert_float_to_s16", referenced from:
      _retro_audio_cb in main.c.o
ld: symbol(s) not found for architecture x86_64

So, I think I need to link to some libretro lib. I will explore this.