Add depend on yaml-cpp for yocs_ar_marker_tracking

`yaml-cpp` is clearly referenced in the `CMakeLists.txt`, so it should be a dependency in the `package.xml`.

Example of build failure due to this bug: http://csc.mcs.sdsmt.edu/jenkins/job/ros-indigo-yocs-ar-marker-tracking_binaryrpm_heisenbug_x86_64/3/console

Thanks,

--scott