Multiple definition of `__vector_9'

[jremington]

@cattledog

The interrupt vector table for the 32u4 shows this for vectors 9 and 10.

Confusingly, the conflict is reported to be with vector 9, which is reserved.

I'm puzzled by this, and wonder if there is a problem with the Arduino Micro board definition, e.g. some confusion about the vector numbering system.