Add missing Mystique vsync interrupt. Helps with some Mystique Win9x drivers.

src/vid_svga.h

@@ -129,6 +129,9 @@ typedef struct svga_t
           vertical line interrupt*/
         int (*line_compare)(struct svga_t *svga);
         
+        /*Called at the start of vertical sync*/
+        void (*vsync_callback)(struct svga_t *svga);
+        
         /*If set then another device is driving the monitor output and the SVGA
           card should not attempt to display anything */
         int override;