msgpack: fix build without threads support

msgpack: fix build without threads support

Some example code depends on pthreads, and we generally don't build
examples in Buildroot, so disable their build.

Fixes:
http://autobuild.buildroot.net/results/f5d/f5d493dbe65cbe58d0924176efbd12e4a078f3ee/
http://autobuild.buildroot.net/results/813/81387649d9dbdfe4ccba9c7d0885aa8bd5239938/
http://autobuild.buildroot.net/results/9d4/9d486fca93136bb9dbf22c9101186d9d1fc0eb66/

Cc: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
[Thomas: completely disable examples.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>