FROMLIST: kheaders: Follow symlinks to source files.

When the kernel is built inside a sandbox container,
a forest of symlinks to the source files may be
created in the container. In this case, the generated
kheaders.tar.xz should follow these symlinks
to access the source files, instead of packing
the symlinks themselves.

Bug: 276339429
Link: https://lore.kernel.org/lkml/20230420010029.2702543-1-elsk@google.com/


Signed-off-by: Yifan Hong <elsk@google.com>
Reviewed-by: Matthias Maennich <maennich@google.com>
Change-Id: Ie1e398ddd342ff2a42418286823f069b03dea254