6relayd: prevent build on AVR32

AVR32 misses a bunch of updated syscalls, including timerfd_create()
which is used by 6relayd. Since that requires Linux/libc changes, just
prevent building it for now.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt@37040 3c298f89-4303-0410-b956-a3cf2f4a3e73