lib/kargs: Don't expose OstreeKernelArgs autoptr cleanup on old glib

lib/kargs: Don't expose OstreeKernelArgs autoptr cleanup on old glib

Move the OstreeKernelArgs autoptr cleanup definition to
ostree-autocleanups.h, which will only expose the definitions when
building ostree or if glib is new enough. The include of
ostree-kernel-args.h needs to be moved before ostree-autocleanups.h in
ostree.h so that the OstreeKernelArgs type is declared when the autoptr
cleanup is defined. All the places it's used already pull in libglnx.h
first so that the compat macros are picked up if glib it too old during
the ostree build.

Closes: #1892
Approved by: jlebon