Instead of including `key_defs.h`, include `KeyboardioFirmware.h` in
`Keyboardio-MouseKeys.h`, so that the dependency is clear. This is
needed for the arduino-builder to arrange the linking order properly
when using KeyboardioFirmware, KeyboardioFirmware-MouseKeys and other
libraries combined. It does not affect the combination of just the two,
but if there's a third, that happens to include `KeyboardioFirmware.h`
too, this dependency is required for correct linking.
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>