🐛 remove global display definition

display was defined globally resulting in linker errors:

/usr/bin/avr-ld: obj/ssd1306.o:[...]ssd1306.h:69: multiple definition of `display'; obj/Keyboard.o: [...]ssd1306.h:69: first defined here /usr/bin/avr-ld: disabling relaxation; it will not work with multiple definitions

As it was only used in ssd1306.c it should be safe to move it out of the header file.