samples: google_iot_mqtt: Explicitly cast socklen_t to int in printf

Otherwise, it can lead to warning like:

main.c:80:10: warning: format '%d' expects argument of type 'int',
but argument 2 has type 'socklen_t' {aka 'long unsigned int'}
[-Wformat=]

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>