null_blk: Fix the WARNING: modpost: missing MODULE_DESCRIPTION()

No functional changes intended.

Fixes: f2298c04 ("null_blk: multi queue aware block test driver")
Signed-off-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20240506075538.6064-1-yanjun.zhu@linux.devSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/main.c

@@ -2135,4 +2135,5 @@ module_init(null_init);
 module_exit(null_exit);
 
 MODULE_AUTHOR("Jens Axboe <axboe@kernel.dk>");
+MODULE_DESCRIPTION("multi queue aware block test driver");
 MODULE_LICENSE("GPL");