Export dm_io_async_bvec so dm-raid1 can find it when built as a module.

--- diff/drivers/md/dm-io.c	2003-12-29 10:12:47.000000000 +0000
+++ source/drivers/md/dm-io.c	2003-12-29 10:15:10.000000000 +0000
@@ -578,3 +578,4 @@
 EXPORT_SYMBOL(dm_io_put);
 EXPORT_SYMBOL(dm_io_sync);
 EXPORT_SYMBOL(dm_io_async);
+EXPORT_SYMBOL(dm_io_async_bvec);