tests: Fix build with old glib
tests: Fix build with old glib

test-checksum.c was using g_autofree without including libglnx.h which
has the backport for that.

Closes: #304
Approved by: cgwalters

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