alsa-gobject: timer: add global method to retrieve current source of timestamp

alsa-project/alsa-gobject pull request #27 was opened from takaswie:

ALSA snd_timer kernel module has a parameter for source of timestamping. Applications cannot refer to the parameter via UAPI of Linux kernel, however it's possible via sysfs.

This patch adds an API to refer to the parameter.

Request URL : https://github.com/alsa-project/alsa-gobject/pull/27 Patch URL : https://github.com/alsa-project/alsa-gobject/pull/27.patch Repository URL: https://github.com/alsa-project/alsa-gobject