kunit: slub: add module description

kunit: slub: add module description

Modules without a description now cause a warning:

WARNING: modpost: missing MODULE_DESCRIPTION() in lib/tests/slub_kunit.o

Link: https://lkml.kernel.org/r/20250324173242.1501003-10-arnd@kernel.org
Fixes: 6c6c1fc09de3 ("modpost: require a MODULE_DESCRIPTION()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Guenetr Roeck <linux@roeck-us.net>
Cc: Jeff Johnson <jeff.johnson@oss.qualcomm.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Pei Xiao <xiaopei01@kylinos.cn>
Cc: Rae Moar <rmoar@google.com>
Cc: Stehen Rothwell <sfr@canb.auug.org.au>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>