thunderbolt: build kunit tests without structleak plugin

The structleak plugin causes the stack frame size to grow immensely when
used with KUnit:

drivers/thunderbolt/test.c:1529:1: error: the frame size of 1176 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

Turn it off in this file.

Linus already split up tests in this file, so this change *should* be
redundant now.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>