MAINTAINERS: Add Clang CFI section

Sami is the primary developer and Kees has been chauffeuring the patches
to Linus so ensure they are always kept in the loop about proposed
changes to these files. Add Nick and I as reviewers so we are CC'd as
well.

Fixes: cf68fffb ("add support for Clang CFI")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210531210629.864888-1-nathan@kernel.org