overlay: Fix build after 646b4a9483

The compile cleanup was not without fault... Apparently I don't have
XVMC enabled anymore and so missed that this variable is actually used.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/i830_video.c

@@ -1440,6 +1440,7 @@ i830_copy_video_data(ScrnInfoPtr scrn, intel_adaptor_private *adaptor_priv,
 		     INT32 x1, INT32 y1, INT32 x2, INT32 y2,
 		     int id, unsigned char *buf)
 {
+	intel_screen_private *intel = intel_get_screen_private(scrn);
 	int srcPitch = 0, srcPitch2 = 0;
 	int top, left, npixels, nlines, size;