linux-libc-headers: kernel headers are installed in STAGING_KERNEL_BUILDDIR

Since 'fido', kernel headers are installed in STAGING_KERNEL_BUILDDIR,
not STAGING_KERNEL_DIR. So this message in the linux-libc-headers
recipe is slightly misleading.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit bdd06f8e)
Signed-off-by: Steve Sakoman <steve@sakoman.com>