gcc-plugins/stackleak: Don't instrument itself

There is no need to try instrumenting functions in kernel/stackleak.c.
Otherwise that can cause issues if the cleanup pass of stackleak gcc plugin
is disabled.

Signed-off-by: Alexander Popov <alex.popov@linux.com>
Link: https://lore.kernel.org/r/20200624123330.83226-2-alex.popov@linux.com


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