ALSA: core: Report audio_tstamp in snd_pcm_sync_ptr

It looks like a simple mistake that this struct member
was forgotten.

Audio_tstamp isn't used much, and on some archs (such as x86) this
ioctl is not used by default, so that might be the reason why this
has slipped for so long.

Fixes: 4eeaaeae ("ALSA: core: add hooks for audio timestamps")
Signed-off-by: David Henningsson <diwic@ubuntu.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Cc: <stable@vger.kernel.org> # v3.8+
Signed-off-by: Takashi Iwai <tiwai@suse.de>