make headers_install can't handle fancy conditions, so let's simplify
things for it a bit.

Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459348943-12803-1-git-send-email-daniel.vetter@ffwll.ch