gcc-plugins/structleak: add makefile var for disabling structleak

[ Upstream commit 554afc3b9797511e3245864e32aebeb6abbab1e3 ]

KUnit and structleak don't play nice, so add a makefile variable for
enabling structleak when it complains.

Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>