avdevice/xcbgrab: fix -Wunused-variable

This patch fixes a -Wunused-variable reported in e.g
http://fate.ffmpeg.org/log.cgi?time=20150918194649&log=compile&slot=x86_64-debian-asan-144800.
av_unused is used as opposed to a header guard for readability.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavdevice/xcbgrab.c

@@ -591,7 +591,7 @@ static void setup_window(AVFormatContext *s)
     uint32_t values[] = { 1,
                           XCB_EVENT_MASK_EXPOSURE |
                           XCB_EVENT_MASK_STRUCTURE_NOTIFY };
-    xcb_rectangle_t rect = { 0, 0, c->width, c->height };
+    av_unused xcb_rectangle_t rect = { 0, 0, c->width, c->height };
 
     c->window = xcb_generate_id(c->conn);