obexftp: Fix crasher due to missing D-Bus threads support

Spotted by Serge Gavrilov <serge@pdmi.ras.ru>

The GDbus port removed the need for us to initialise
libdbus' threads support, and commit
7d4bd613
removed the last call to it.

However, the obexftp backend still hasn't fully been ported
to GDbus, and needs this threading support. Re-add it directly
in the backend.

https://bugzilla.gnome.org/show_bug.cgi?id=693574