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

Steps to reproduce:
 - enter-chroot in a first crosh
 - startxfce in a second crosh, then press Ctrl-C

ps -eaf shows that xbindkeys is still running. However, it's 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 bugs reported on Google+.