sys-kernel/linux-headers: bump up to 5.1

sys-kernel/linux-headers: bump up to 5.1

This is the first gentoo release where we don't use rip-headers.sh
and use upstream tarball directly to provide us headers.

It will increase time and temporary space to unpack a tarball
but it should be comparable or smaller than other toolchain
packages like gcc or glibc. It's a reasonable price not to
maintain our header ripping tools. More maintainable solution
would be for upstream to provide headers-dist Makefile target
(and tarballs).

Reported-by: jamesrutledge@earthlink.net
Closes: https://bugs.gentoo.org/686892
Package-Manager: Portage-2.3.67, Repoman-2.3.15
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>