tree-wide: Use g_autoptr(Ostree*)

Part of cleaning up our usage of libglnx; we want to use what's in GLib where we
can.

Had to change a few .c files to `#include ostree.h` early on to pick up
autoptrs for the core types.

Closes: #1040
Approved by: jlebon

src/libostree/ostree-bootloader.h

@@ -48,6 +48,7 @@ struct _OstreeBootloaderInterface
                                                    GError       **error);
   gboolean             (* is_atomic)              (OstreeBootloader  *self);
 };
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (OstreeBootloader, g_object_unref)
 
 GType _ostree_bootloader_get_type (void) G_GNUC_CONST;