ALSA: pcm: Unexport snd_pcm_lib_preallocate_pages_free*()

snd_pcm_lib_preallocate_pages_free() is no longer used, so drop it.
snd_pcm_lib_preallocate_pages_free_for_all() is called only from PCM
core, so unexport and move the declaration into the local header.

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