Documentation: Avoid duplicate Kconfig inclusion

Documentation/Kconfig is already included from top-level, avoid
including it again from lib/Kconfig.debug.

Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20230114-doc-v2-1-853a8434ac95@pefoley.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>