net: sntp: Allow to build for CONFIG_POSIX_API.
net: sntp: Allow to build for CONFIG_POSIX_API.

Use POSIX headers in this case.

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr