gpsd: disallow NMEA2000 CAN driver on AVR32

AVR32 is using uClibc 0.9.31, which lacks some CAN related definitions
that prevent the gpsd nmea2000 driver from building.

The rest of gpsd, with all options and drivers enabled, builds fine on
AVR32.

Fixes:

  http://autobuild.buildroot.org/results/ee2/ee2ec848e893f08fa80caf99a67e68b73b6400e8/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>