Add libvroom example to CI builds

[jcoupey]

Every now and then, we do some refactoring of the C++ API or internal data structures that break the build for libvroom_examples/libvroom.cpp. This usually go unnoticed until someone reports it like in #510 so it would be nice to add a simple build check in the CI setup.

Note: this is not about treating failed builds as an error as we don't provide any guarantee of stability of the C++ API, but rather a way to spot changes so that the example stays in sync with core updates.