Add set(CMAKE_DEBUG_POSTFIX "d")

[traversaro]

This is useful to correctly link in Windows Visual Studio.

[traversaro]

Relevant YARP code:

• https://github.com/robotology/yarp/blob/9a395f6e41e01d8d350b6a4e5e2b2f6c6f913b74/doc/yarp_build_structure.dox#L207
• https://github.com/robotology/yarp/blob/0a8aaa2482d029d5668f2482f29ea4b882d0d17d/cmake/YarpSystemCheck.cmake#L197