arptables: bump to 2015-05-20

arptables: bump to 2015-05-20

This fixes building with musl and drops the dependency on the OpenWrt
kernel-header patches:

  270-uapi-kernel.h-glibc-specific-inclusion-of-sysinfo.h.patch
  271-uapi-libc-compat.h-do-not-rely-on-__GLIBC__.patch
  272-uapi-if_ether.h-prevent-redefinition-of-struct-ethhd.patch

Use the new upstream location at netfilter.org and use a define instead
of a patch to "optimize".

See also: https://git.netfilter.org/arptables/log/

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
[Jo-Philipp Wich: add mirror SHA256 sum]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>