xen/blkback: add missing MODULE_DESCRIPTION() macro

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/block/xen-blkback/xen-blkback.o

Add the missing invocation of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20240602-md-block-xen-blkback-v1-1-6ff5b58bdee1@quicinc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: jeff-t-johnson

drivers/block/xen-blkback/blkback.c

@@ -1563,5 +1563,6 @@ static void __exit xen_blkif_fini(void)
 
 module_exit(xen_blkif_fini);
 
+MODULE_DESCRIPTION("Virtual block device back-end driver");
 MODULE_LICENSE("Dual BSD/GPL");
 MODULE_ALIAS("xen-backend:vbd");