linux_testing (5.2.0-rc1): fix build, include 'cpio' in nativeBuildInputs

81d4e658 automatically bumped
linux_testing to 5.2.0-rc1, but the 5.2 merge window included a new
feature adding compressed headers for compiled kernels into
/proc/kheaders.tar.xz

See https://github.com/torvalds/linux/commit/43d8ce9d65a54846d378545770991e65838981e0



This feature requires 'cpio' to now be included in nativeBuildInputs
since it's used to construct that archive.

This wasn't caught by Hydra since we turn off build of linuxPackages,
but ideally we should at least build the kernel in the future
(linux_testing itself.)

Signed-off-by: Austin Seipp <aseipp@pobox.com>