bat: Don't pass incompatible function pointers to pthread_cleanup_push()

bat: Don't pass incompatible function pointers to pthread_cleanup_push()

pthread_cleanup_push() takes a function pointer for void (void *).
Although it may work in most cases, we shouldn't pass an incompatible
function pointer there, as some old gcc complains:
  alsa.c:560: warning: initialization from incompatible pointer type
  alsa.c:562: warning: initialization from incompatible pointer type

Signed-off-by: Takashi Iwai <tiwai@suse.de>