MAINTAINERS: hardening: Add Gustavo as Reviewer

It's an oversight to not have already listed Gustavo here. Add him as a
Reviewer.

Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

MAINTAINERS

@@ -11393,6 +11393,7 @@ F:	usr/
 
 KERNEL HARDENING (not covered by other areas)
 M:	Kees Cook <keescook@chromium.org>
+R:	Gustavo A. R. Silva <gustavoars@kernel.org>
 L:	linux-hardening@vger.kernel.org
 S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git for-next/hardening