Template of libdnf5 plugin is not possible to compile with current version of DNF5/libdnf5

[jirihnidek]

The template of libdnf5 plugin described on this web page:

https://dnf5.readthedocs.io/en/latest/templates/template-libdnf5-plugin.html

is not possible to compile with current version of libdnf5.

The CmakeFiles.txt is also intended for plugins that are part of dnf5 repository.

I created my own repository with this template to demonstrate these issues and I created PR with unmodified template.cpp:

jirihnidek/libdnf5-plugin-template#1

and then I created another PR showing what is necessary to change in the code:

jirihnidek/libdnf5-plugin-template#2

[kontura]

Thank you for the report, we should fix that.
We should also do #2016 to make sure this doesn't keep happening.