[xmonad] bump X11 dependency

A corresponding patch just went into xmonad-contrib. Thu Mar 15 20:01:06 EDT 2012 Daniel Wagner * bump X11 dependency Strictly speaking, it probably isn't necessary to depend on a newer X11, since xmonad still builds just fine with X11-1.5. However, xmonad-contrib does *not* build fine with X11-1.5, and we would like to avoid the confusing situation where someone successfully builds xmonad (with X11-1.5), then finds that building xmonad-contrib requires rebuilding xmonad from scratch (with a newer X11).