kunit/usercopy: Add missing MODULE_DESCRIPTION()

Fix warning seen with:

$ make allmodconfig && make W=1 C=1 lib/usercopy_kunit.ko
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/usercopy_kunit.o

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>