Cleanup: [CMake] don't add strgen.h twice

[glx22]

Motivation / Problem

When doing some testing with CMake to prevent duplicate filenames, I found 1> [CMake] openttd_lib: D:/developpement/GitHub/glx22/OpenTTD/src/strgen/strgen.h filename is a duplicate of D:/developpement/GitHub/glx22/OpenTTD/src/strgen/strgen.h
It's because in src we add strgen/strgen.h while in src/strgen we already added strgen.h

Description

Remove the useless add.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker