gnu: xfce4-whiskermenu-plugin: Fix installed shell script.

* gnu/packages/xfce.scm (xfce4-whiskermenu-plugin): Fix installed shell script.
[arguments]: Add phase fix-shell-script to substitute paths to xfce4-panel and
gettext.
[inputs]: Add gettext-minimal.

Co-authored-by: Kei Kebreau <kkebreau@posteo.net>
Signed-off-by: Kei Kebreau <kkebreau@posteo.net>