In commit 197cb419d1 (add_custom_command: Add CODEGEN support,
2024-05-27, v3.31.0-rc1~394^2) we accidentally left out the global
`codegen` target's dependencies on the per-directory `codegen` targets.
Add them for parity with the `all` target.
Fixes: #26517
By specifying CODEGEN as an argument to add_custom_command the
custom command will be added to a codegen build target.
The intent is to provide a convenient way for users to get
their generated files without having to build the whole project.
This can be helpful for code analysis tools which can be useful
for IDEs and CI.