zmqpp: fix build failure

The zmqpp client needs a little help to link against libpthread. Fix build
failures such as the following.

  http://autobuild.buildroot.net/results/d0a/d0a740d4d5414844ca7f2de266fc92223d3d20c8



Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>