vnc: fix build error from VNC_DIRTY_WORDS Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

diff --git a/ui/vnc.c b/ui/vnc.c
index 2f71769..14f2930 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c

@@ -1659,7 +1659,7 @@
         for (i = 0; i < h; i++) {
             bitmap_set(vs->dirty[y_position + i], 0, width);
             bitmap_clear(vs->dirty[y_position + i], width,
-                         VNC_DIRTY_WORDS * BITS_PER_LONG - width);
+                         VNC_DIRTY_BITS - width);
         }
     }
 }