If you cannot include the whole mysensors header, you maybe can just forward declare the function. (basically, just copy the line from the mysensors.h in your mysEeprom.cpp file.
/*extern*/ void saveState(uint8_t pos, uint8_t value);
// extern probably not needed, that's real old C, but you I have had
// surprises with the arduino compiler… You might need to add it.
What's the issue with namespaces or classes?