libpfm4: disable on avr32

The perf_event_open syscall is not available on avr32. Fixes build failures
such as the following.

  http://autobuild.buildroot.net/results/38f/38fa81315be448c409b9af29f92621fcf5676f71/



Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>