seq: user_client: use constant qualifier for read-only parameter

alsa-project/alsa-gobject pull request #20 was opened from takaswie:

The 'port_id' parameters of 'alsaseq_user_client_create_port()' is a pointer to storage of unsigned char type, due to optional parameter in g-i interface. It's read-only if having value and it's better to have const qualifier.

Request URL : https://github.com/alsa-project/alsa-gobject/pull/20 Patch URL : https://github.com/alsa-project/alsa-gobject/pull/20.patch Repository URL: https://github.com/alsa-project/alsa-gobject