ALSA: memalloc: Move snd_malloc_ops definition into memalloc.c again

ALSA: memalloc: Move snd_malloc_ops definition into memalloc.c again

The definition of struct snd_malloc_ops was moved out to
memalloc_local.h since there was another code for S/G buffer
allocation referring to the struct.  But since the code change to use
non-contiguous allocators, it's solely referred in memalloc.c, hence
it makes little sense to have a separate header file.

Let's move it back to memalloc.c.

Link: https://patch.msgid.link/20240910113141.32618-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>