tests: net: socket: Add correct path to socket.h for POSIX_API

If CONFIG_POSIX_API is enabled, then the socket.h is found under
zephyr/posix/sys/socket.h etc. This allows one to compile the
socket test applications without error prints.

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