gnu: xonsh: Add missing input.

Xonsh vim emulation mode/environment variable does not work without this.

* gnu/packages/shell.scm (xonsh)[inputs]: Add PYTHON-PROMPT-TOOLKIT.

Signed-off-by: Nicolas Goaziou <mail@nicolasgoaziou.fr>

gnu/packages/shells.scm

@@ -580,6 +580,7 @@ ksh, and tcsh.")
     (inputs
      (list python-distro
            python-ply
+           python-prompt-toolkit
            python-pygments
            python-pyperclip
            python-setproctitle))