bug#41049: LyX: No application icon

[sirgazil]

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

## Steps to reproduce

1. guix install lyx
2. Press Alt + F2, type r and hit Enter to restart GNOME Shell (see 
http://issues.guix.gnu.org/issue/35594)
3. Got to Activities → Show Applications (nine dots button) → All


# Expected result

The LyX application icon should be listed.


# Unexpected result

The LyX application icon is not listed.


# 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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