block: avoid a write only variable

Compiling with GCC 4.6.0 20100925 produced a warning:
/src/qemu/block/qcow2-refcount.c: In function 'update_refcount':
/src/qemu/block/qcow2-refcount.c:552:13: error: variable 'dummy' set but not used [-Werror=unused-but-set-variable]

Fix by adding a dummy cast so that the result is not unused.

Acked-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

block/qcow2-refcount.c

@@ -551,6 +551,7 @@ fail:
     if (ret < 0) {
         int dummy;
         dummy = update_refcount(bs, offset, cluster_offset - offset, -addend);
+        (void)dummy;
     }
 
     return ret;