xapp_xbacklight: select xcb-util package

xapp_xbacklight depends on the xcb-utils package but doesn't select it.
Fixes:

http://autobuild.buildroot.net/results/d96/d96aa77e02954bef6fb6e9d244fbf9d1e939a374/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>