bug#41050: LyX: No icons in toolbars

[sirgazil]

This happens in both master and core-updates. Also in both real and virtual 
machines.

## Steps to reproduce

1. guix install lyx
2. Press Alt + F2, type lyx2.3 and hit Enter to start LyX


# Expected result

LyX starts and the toolbar buttons should have icons (see 
https://www.lyx.org/Screenshots).


# Unexpected result

The icons are missing (see 
https://multimedialib.files.wordpress.com/2020/05/guix-lyx-missing-icons-in-toolbars-2020-05-03.png).


# System information

```
$ lyx2.3 --version
LyX 2.3.3 (2019-06-10)
CMake Build

$ LANG=C guix describe
Generation 78   May 02 2020 18:00:20    (current)
  guix 7b07852
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: core-updates
    commit: 7b07852ddb334c92bcef69666f21c599f1f0fa79
  sirgazil-x ef6ac93
    repository URL: https://gitlab.com/sirgazil/guix-channel-x.git
    branch: master
    commit: ef6ac9331a7847fdec6f0eb199524b1b755ba0df
```


---
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