Fix compilation of xcbgrab if xcb/shape.h is not available.

Old versions of libxcb do not provide xcb_shape_rectangles().
The issue can be fixed differently but this small change fixes
some fate platforms and a user reported compilation problem.

Reported and tested by trac user kevmitch.
Fixes ticket #4067.

libavdevice/xcbgrab.c

@@ -580,11 +580,13 @@ static void setup_window(AVFormatContext *s)
                       XCB_COPY_FROM_PARENT,
                       mask, values);
 
+#if XCB_SHAPE_RECTANGLES
     xcb_shape_rectangles(c->conn, XCB_SHAPE_SO_SUBTRACT,
                          XCB_SHAPE_SK_BOUNDING, XCB_CLIP_ORDERING_UNSORTED,
                          c->window,
                          c->region_border, c->region_border,
                          1, &rect);
+#endif
 
     xcb_map_window(c->conn, c->window);