scripts/kernel-doc: add modeline for vim users

scripts/kernel-doc: add modeline for vim users

Set 'softtabstop' to 4 spaces, which will hopefully help keep the
indentation in this file consistent going forwards.

This mirrors the modeline in scripts such as recordmcount.pl, ktest.pl,
and others.

Emacs seems to use 4 spaces to indent by default, so it doesn't require
anything special here.

No functional change.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240215134828.1277109-2-vegard.nossum@oracle.com