sntp-client: Add library if using FreeRTOS-Plus-TCP stack

sntp helper library depends on FreeRTOS-Plus-TCP connectivity
stack as it includes `FreeRTOS_IP.h` header file in one of its
source files (sntp_client_task.c), thus this library is only added
in case of using FREERTOS_PLUS_TCP connectivity stack.

Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>