MAINTAINERS: Include stackleak paths in hardening entry

While most of the gcc-plugins are self-contained in the
scripts/gcc-plugins directory, stackleak actually has some additional
files. Add those so changes are directed to the hardening list.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20231019004616.work.960-kees@kernel.org


Signed-off-by: Kees Cook <keescook@chromium.org>