Documentation: Kunit: add MODULE_LICENSE to sample code

The sample code has Kconfig for tristate configuration. In the case, it
could be friendly to developers that the code has MODULE_LICENSE, since
the missing MODULE_LICENSE brings error to modpost when the code is built
as loadable kernel module.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>