lib: fortify_kunit: build without structleak plugin

Building allmodconfig with aarch64-linux-gnu-gcc (Debian 11.3.0-6),
fortify_kunit with strucleak plugin enabled makes the stack frame size
to grow too large:

lib/fortify_kunit.c:140:1: error: the frame size of 2368 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]

Turn off the structleak plugin checks for fortify_kunit.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Kees Cook <keescook@chromium.org>