dbus: requires thread support

As can be seen on the build result at
http://autobuild.buildroot.org/results/20f1078ef7dc5f187b04c63ef70e8b43acf9bb3a/build-end.log

,
D-Bus requires thread support in the toolchain.

This commit adjusts the Kconfig dependencies of D-Bus and all its
reverse dependencies to depend on thread support in the toolchain.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>