yaml-cpp: requires boost

The Boost C++ library is already selected in the Config.in but it was
missing in the yaml-cpp.mk file.

Fix build failure:
CMake Error at /home/tetsuya/buildroot/br/output/host/usr/share/cmake-3.1/Modules/FindBoost.cmake:1182 (message):
  Unable to find the requested Boost libraries.
<snip>
make: *** [/home/tetsuya/buildroot/br/output/build/yaml-cpp-0.5.1/.stamp_configured] Error 1

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>