croutonxinitrc-wrapper: Make sure xbindkeys is killed on exit.

Steps to reproduce:
 - `enter-chroot` in a first crosh
 - `startxfce4` in a second crosh, then press Ctrl-C.

`ps -eaf` shows that xbindkeys is still running. However, its parent is now PID 1, and it received a SIGHUP (due to the parent shell process terminating).
Unfortunately, xbindkeys does not handle SIGHUP correctly, and hangs forever (locked in a `futex` call according to ptrace: did not investigate further), and does not process any further key presses.

This might explain #754, and some similar bugs reported on Google+.