drawPixel() fixed to match Adafruit_GFX

Using the latest version of Adafruit_GFX, the method signature of the implementation of drawPixel() in EPD_GFX.h does not match the virtual method signature of drawPixel() in Adafruit_GFX.h. This results in compile time errors about the drawPixel() method not being implemented. I copied the signature from Adafruit_GFX.h into EPD_GFX.h and confirmed that everything compiled and ran properly.