Error Compiling for board Arduino Nano while using Adafruit GFX
Currently, you defined a forward declaration for drawLine, however I assume you want to call the method (function) inside display.
So change
drawLine(20,60,5,50,256);

into
display.drawLine(20,60,5,50,256);

And you can remove the forward declaration, because this method is already defined in the library.
void drawLine(uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1, uint16_t color);

Note, if you call drawLine without an instance (which is display), then the linker tries to find the drawLine function which it cannot find, so you get a linker error (see ld returned in your error, together with undefined reference).