dbus: fix static linking issue with pthread

-lpthread is missing when dbus is build with static lib

Fixes build failures:
http://autobuild.buildroot.net/results/80a317e33afd1d5ba2f7dd9ecd89314b09b4b1fe/
http://autobuild.buildroot.net/results/2c3c040624652547bbeec363d7d6b03f2477ff00/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>