Mesos configured with `--enable-grpc` doesn't compile on non-Linux builds

Commit 59cca968e04dee069e0df2663733b6d6f55af0da added examples/test_csi_plugin.cpp to non-Linux builds that are configured using the --enable-grpc flag. As examples/test_csi_plugin.cpp includes fs/linux.hpp, it can only compile on Linux and needs to be disabled for non-Linux builds.