net: sockets_service: Add build check for using zephyr libc

Add checks to make sure that we are not trying to use the socket
service library with eventfd if CONFIG_POSIX_API is not set and if
using native_sim based board. The reason is that we should always
use zephyr libc based eventfd implementation instead of host libc one.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>