net: provide defines for _POLICY_FAILED until all code is updated

Upstream introduced ICMPV6_POLICY_FAIL for code 5 of destination
unreachable, conflicting with our name.

Add appropriate defines to allow our code to build with the new
name until we have updated our local patches for older kernels
and userspace packages.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>