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>

Documentation/dev-tools/kunit/start.rst

@@ -250,6 +250,8 @@ Now we are ready to write the test cases.
 	};
 	kunit_test_suite(misc_example_test_suite);
 
+	MODULE_LICENSE("GPL");
+
 2. Add the following lines to ``drivers/misc/Kconfig``:
 
 .. code-block:: kconfig