create-spdx: Add kernel work-shared source

Since the kernel source is stored in work-shared, we need to add it
to the search path so the kernel and kernel-modules source code can
be found correctly.

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>