bindings: cxx: mark public classes as final

bindings: cxx: mark public classes as final

libgpiod C++ types are not meant to be inherited from. They already don't
even provide virtual destructors so mark them explicitly as final. With
that the destructors of throwable types can also be made non-virtual.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>