bindings: cxx: fix event timestamp calculation for 32bit

bindings: cxx: fix event timestamp calculation for 32bit

Use appropriate C++ chrono library functions to convert the event
timestamp from a struct timespec to ::std::chrono::nanoseconds to
ensure correct conversion independent of platform.

Fixes: 8078a4a2ad90 ("bindings: implement C++ bindings")
Reported-by: Florian Evers <florian-evers@gmx.de>
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>