firewire: fix build failure due to missing module license

The added KUnit test has no MODULE_LICENSE even if built for tristate. It
brings build failure in linux-next integration.

This commit releases the test under GPL and fixes the bug.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/lkml/20230530122450.1603af75@canb.auug.org.au/
Fixes: dc7c516 ("firewire: add KUnit test to check layout of UAPI structures")
Link: https://lore.kernel.org/r/20230530102532.56386-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>

Author: takaswie

drivers/firewire/uapi-test.c

@@ -85,3 +85,5 @@ static struct kunit_suite structure_layout_test_suite = {
 	.test_cases = structure_layout_test_cases,
 };
 kunit_test_suite(structure_layout_test_suite);
+
+MODULE_LICENSE("GPL");