fix compiler's warnings

[JM-FRANCE]

When compiling Adafruit_SPITFT.cpp on AVR boards the Adafruit_SPITFT::writePixels member function does not use block nor bigEndian.

Suggest to use attribute ((unused)) and changing the signature to

void Adafruit_SPITFT::writePixels(uint16_t *colors, uint32_t len, __attribute__ ((unused)) bool block, __attribute__ ((unused)) bool bigEndian) {

to avoid compiler warnings.

[ladyada]

sounds good - please submit a PR if you can it would be appreciated! :)

[Mark1626]

I think this got closed by #275, if not I can try this as my first issue here