lwip/sntp: Added extern "C"
lwip/sntp: Added extern "C"

so that sntp_set_time_sync_notification_cb can be used from c++

Signed-off-by: Konstantin Kondrashov <konstantin@espressif.com>

Merges: https://github.com/espressif/esp-idf/pull/3700

espressif/esp-idf,espressif/esp-idf,espressif/esp-idf,espressif/esp-idf