Added gtk2-engines-pixbuf to remove warnings in XFCE

[andzaytsev]

I get the following warning messages on the xfce target after sudo startxfce4:

(xfsettingsd:21732): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",
(xfsettingsd:21732): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",
(xfdesktop:21724): Gtk-WARNING **: Unable to locate theme engine in module_path: "pixmap",

According to this askubuntu question, one should install gtk2-engines-pixbuf. It is included by default in the gnome target but not the xfce target. Installing the package fixed the warnings for me.

I think gtk2-engines-pixbuf should be installed on xfce by default.

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[andzaytsev]

I signed it!

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CLAs look good, thanks!

[dnschneid]

So, the place you added the package actually tells crouton to avoid installing the package. You need to add it prior to the --

[zguithues]

@andzaytsev @dnschneid Does anyone have an objection to me fixing this pull request and resubmitting? Seems like a simple fix & I'm trying to get more familiar with things like this.

[dnschneid]

@zguithues go for it, it seems :)