xwm: remove xcb-util dependency
xwm: remove xcb-util dependency

This hardcodes a define, but other projects (wlc/weston) either use the raw
number or just redefine it with another name anyway...

This should fix travis build.

swaywm/wlroots,swaywm/wlroots,SirCmpwn/wlroots,ascent12/wlroots,ascent12/wlroots