scripts/kernel-doc: Use $KBUILD_BUILD_TIMESTAMP as man page date
scripts/kernel-doc: Use $KBUILD_BUILD_TIMESTAMP as man page date

Together with the preceding changes, this allows man pages to be built
reproducibly.

Signed-off-by: Ben Hutchings <73675debcd8a436be48ec22211dcf44fe0df0a64@decadent.org.uk>
Signed-off-by: Jonathan Corbet <b2ce64d5587c02f12e367eac59751145a0660c51@lwn.net>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs