Add INT2 case to detachInterrupt()

[leres]

detachInterrupt() is missing the INT2 case, needed for chips such as the atmega644p.

Here's a patch that adds the necessary code when INT2 is defined:

extrapatch-hardware-arduino-cores-arduino-WInterrupts.c

[vicatcu]

I just submitted pull request #3221 to address this

[cmaglie]

Fixed with #3221, thanks!