add waitvsync() for atari and atari5200

include/atari5200.h

@@ -89,5 +89,8 @@ extern void atr5200std_joy[];        /* referred to by joy_static_stddrv[] */
 */
 #define _bordercolor(color) 0
 
+extern void waitvsync (void);
+/* Wait for start of next frame */
+
 /* End of atari5200.h */
 #endif