docs: conf.py: Ignore __counted_by attribute

It seems that Sphinx is confused by the __counted_by attribute on struct
members. Add it to the list of known attributes.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312150614.kOx8xUkr-lkp@intel.com/
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-doc@vger.kernel.org
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231215001347.work.151-kees@kernel.org