samples: tflite-micro: hello_world: Enable standard C++ library

The TensorFlow Lite module makes use of the features provided by the
standard C++ library (e.g. `#include <functional>`), so the standard
C++ library config must be enabled for it.

This used to work without `CONFIG_LIB_CPLUSPLUS=y` due to the bug
described in the issue #36644.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>