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

--- diff/drivers/md/dm-io.c	2003-11-26 10:18:32.000000000 +0000
+++ source/drivers/md/dm-io.c	2003-11-26 10:19:45.000000000 +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);