libostree: bump to version 2018.9.1

libostree: bump to version 2018.9.1

Bump to v2018.9 added ostree-finalize-staged.path through
https://github.com/ostreedev/ostree/commit/ac1a919ffd4fe944d06c4f4510604baa73d1bf8e

However, this file was not added in the tarball until commit:
https://github.com/ostreedev/ostree/commit/62594765b2fae72e332ffb21fbd11085d911ef83

So use the new 2018.9.1 tarball to get this file otherwise build with
systemd will fail

Fixes:
 - http://autobuild.buildroot.org/results/fc6435282630e60927f88bf5954805ebbaa60cb3

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>