AvrAdc: Don't fail to compile with no ADC pins.
AvrAdc: Don't fail to compile with no ADC pins.

johnkingsley/aprinter,ambrop72/aprinter,fixator/aprinter,ambrop72/aprinter,fixator/aprinter,ambrop72/aprinter,fixator/aprinter,fixator/aprinter,fixator/aprinter,ambrop72/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,fixator/aprinter