[GLib] Use {class_name}_get_instance_private

[asfimport]

Because G_TYPE_INSTANCE_GET_PRIVATE has been deprecated since version 2.58.

https://developer.gnome.org/gobject/stable/gobject-Type-Information.html#G-TYPE-INSTANCE-GET-PRIVATE

Reporter: Yosuke Shiro / @shiro615
Assignee: Yosuke Shiro / @shiro615

PRs and other links:

• GitHub Pull Request #3017

_{Note: This issue was originally created as ARROW-3858. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 3017
#3017