package/libostree: libfuse is optional, not mandatory

libfuse is optional since its addition in version 2016.2 with
https://github.com/ostreedev/ostree/commit/e9ccdd2d007801ef25cc7283188942d791889c27



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>