libev: disable on avr32

Fixes:

  http://autobuild.buildroot.org/results/4951121337fad670d8bf869483774ccf9e91dd47/build-end.log

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/libev/Config.in

@@ -1,5 +1,7 @@
 config BR2_PACKAGE_LIBEV
 	bool "libev"
+	# needs memory fences
+	depends on !BR2_avr32
 	help
 	  Userspace library for handling asynchronous notifications