extract bionic kernel headers

This patch correctly extracts bionic kernel headers instead
of those under external/kernel-headers/original/.

From external/kernel-headers/original/README.txt:
  This directory contains the original kernel headers that are used to generate Bionic's "cleaned-up" user-land headers.
  They are mostly covered by the GPLv2 + exception, and thus cannot be distributed as part of the platform itself.
  (NOTE: The cleaned up headers do not contain copyrightable information and are distributed with Bionic)