croutonxinitrc-wrapper: Fix xbindkeys kill with dash

`wait "$!"` returns a >0 exit code when interrupted by a signal.
bash still executes the signal handler, then quits (-e), while
dash bails out immediately, leaving xbindkeys with PID 1 as parent.
Merge pull request #879 from drinkcat/fix-xbindkeys-kill

croutonxinitrc-wrapper: Fix xbindkeys kill with dash