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>

package/libpfm4/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_LIBPFM4
 	bool "libpfm4"
+	depends on !BR2_avr32 # no perf_event_open
 	help
 	  A helper library to help encode Performance Events to use with
 	  Operating system kernels performance monitoring interfaces.