lib/repo-file: Add casts to appease GLib g_object_ref cast PR
lib/repo-file: Add casts to appease GLib g_object_ref cast PR

This fixes the build with https://bugzilla.gnome.org/show_bug.cgi?id=790697

Closes: #1363
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree