docs: automarkup.py: Skip C reserved words when cross-referencing

With the transition to Sphinx 3, new warnings were caused by
automarkup, exposing bugs in the name matching.

When automarkup parsed a text like "struct struct" in the documentation,
it tried to cross-reference to a "struct" symbol, which is recognized as
a C reserved word by Sphinx 3, generating a warning.

Add some C reserved words (only the ones that were causing warnings) to
a list and skip them while trying to cross-reference.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@protonmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>