bug#51968: Missing dependency for Xournal++

["Daniel Meißner via Bug reports for GNU Guix" <bug-guix@gnu.org>]

Hi Guix,

there is a problem with the package xournalpp on Guix master.  To
reproduce run:

--8<---------------cut here---------------start------------->8---
$ guix shell --pure xournalpp -- xournalpp
** Message: 10:08:54.258: TEXTDOMAINDIR = (null), PACKAGE_LOCALE_DIR = /gnu/store/dw4ca44bncb2cl0q7yx53lh1md90dcwg-xournalpp-1.1.0/share/locale, chosen directory = /gnu/store/dw4ca44bncb2cl0q7yx53lh1md90dcwg-xournalpp-1.1.0/share/locale
ALSA lib pcm_dsnoop.c:638:(snd_pcm_dsnoop_open) unable to open slave
ALSA lib pcm.c:2660:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.rear
ALSA lib pcm.c:2660:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.center_lfe
ALSA lib pcm.c:2660:(snd_pcm_open_noupdate) Unknown PCM cards.pcm.side
ALSA lib pcm_route.c:877:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_route.c:877:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_route.c:877:(find_matching_chmap) Found no matching channel map
ALSA lib pcm_dsnoop.c:638:(snd_pcm_dsnoop_open) unable to open slave
** Message: 10:08:54.289: Plugin "MigrateFontSizes" UI initialized
**
Gtk:ERROR:gtkiconhelper.c:494:ensure_surface_for_gicon: assertion failed (error == NULL): Icon 'image-missing' not present in theme Adwaita (gtk-icon-theme-error-quark, 0)
Bail out! Gtk:ERROR:gtkiconhelper.c:494:ensure_surface_for_gicon: assertion failed (error == NULL): Icon 'image-missing' not present in theme Adwaita (gtk-icon-theme-error-quark, 0)

** (xournalpp:2068): WARNING **: 10:08:54.371: [Crash Handler] Crashed with signal 6

** (xournalpp:2068): WARNING **: 10:08:54.371: [Crash Handler] Wrote crash log to: /home/daniel/.cache/xournalpp/errorlogs/errorlog.20211119-100854.log

** (xournalpp:2068): WARNING **: 10:08:54.478: Trying to emergency save the current open document?

** (xournalpp:2068): WARNING **: 10:08:54.478: Successfully saved document to "/home/daniel/.config/xournalpp/emergencysave.xopp"
--8<---------------cut here---------------end--------------->8---

Alexander Asteroth, who discovered the problem, wrote to help-guix:
> I found the reason. Xournal++ needs gnome-icon-theme to be installed
> as well. So it should probably go into the dependencies of the
> package.

This problem is easily missed when installing Xournal++ on a Guix system
with Gnome as DE.

Best
Daniel