portal: Add g_autoptr() defines for old GLib versions
portal: Add g_autoptr() defines for old GLib versions

We don't need to check for GLib 2.44 (the first release with g_autoptr()
support) since Flatpak requires that version in configure.ac.

Fixes https://github.com/flatpak/flatpak/issues/3403

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak