docs: deprecated.rst: Add uninitialized_var()

Nothing should be using this macro, and the entire idea of tricking the
compiler into silencing such warnings is a mistake.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: Joe Perches <joe@perches.com>
Cc: linux-doc@vger.kernel.org
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>