community/libxmlb: upgrade to 0.3.9

Mainly maintenance:

Version 0.3.9
~~~~~~~~~~~~~
Released: 2022-05-24

Bugfixes:
 - Fix the crash for when the root tree has no children (Richard Hughes)
 - Fix the crash when getting the element for the [empty] root (Richard Hughes)
 - Install xb-tool into bindir (Richard Hughes)

Version 0.3.8
~~~~~~~~~~~~~
Released: 2022-03-22

Bugfixes:
 - Fix building on macOS (Bobby Rong)

Now xb-tool has a manual page. I wondered if that should be also moved to the -dev subpackage.