You guessed right with this latest one - setSyncProvider is a function defined in the PaulStoffregen/Time library in Time.cpp on line #312:
https://github.com/PaulStoffregen/Time/ ... e.cpp#L312
I've just pulled the latest from github for the time library and verified that iD8266 builds successfully with that here and it does so I'm fairly sure this build error is an Arduino IDE library reference / path issue to Time.h and Time.cpp