Previously, some IDF FreeRTOS API additions that were meant to be private were
exposed through the same "idf_additions.h" header. This commit moves those
functions to a separate header included via
"esp_private/freertos_idf_additions_priv.h" so that they are not mistaken as
public API by users.
This commit also fixes some missing include and C++ guards in idf_additions.h
