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>