samples: chre: Correct main() function prototype

C++ standard requires C++ main() to have the return type of 'int'.

This commit updates the CHRE sample to define main() as
`int main(void)` and enable  `CONFIG_CPP_MAIN`, which instructs the
Zephyr kernel to call the C++ main().

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>