chore(lvgl_private): remove unnecessary private header file includes

[FASTSHIFT]

Description of the feature or fix

Hi, we upgraded LVGL on our system. Our CI encountered the problem that the upper-level application based on LVGL reported that ft2build.h (freetype compilation configuration file) could not be found.

After investigation, we found that lv_freetype_private.h was also exposed to the public space in lvgl_private.h (see this PR: #5913). The original design was that lv_freetype_private.h was only used internally in freetype porting layer, so only the directory where ft2build.h was located was exposed to LVGL. If lv_freetype_private.h was exposed to the public space, the compilation system would need to open the ft2build.h directory to all applications that use lvgl_private.h (and these programs originally only wanted to access display and indev), which is very likely to cause naming pollution and compilation conflicts!!!

I think lvgl_private.h should only contain necessary modules, rather than adding all *_private.h files for the sake of convenience. For new special requirements, we should package them into carefully designed public APIs, and eventually guide applications to gradually move away from lvgl_private.h.

Notes

• Update the Documentation if needed.
• Add Examples if relevant.
• Add Tests if applicable.
• If you added new options to lv_conf_template.h run lv_conf_internal_gen.py and update Kconfig.
• Run scripts/code-format.py (astyle version v3.4.12 needs to be installed) and follow the Code Conventions.
• Mark the Pull request as Draft while you are working on the first version, and mark is as Ready when it's ready for review.
• When changes were requested, re-request review to notify the maintainers.
• Help us to review this Pull Request! Anyone can approve or request changes.

[liamHowatt]

We are preparing to take the private convention much further in #6068.

I can appreciate removing Free type and Windows, but cache too?

CC @kisvegabor

[FASTSHIFT]

We are preparing to take the private convention much further in #6068.

I can appreciate removing Free type and Windows, but cache too?

CC @kisvegabor

I suggest deleting it first, and then adding it if the user really needs it.
cc @W-Mai

[kisvegabor]

I see the problem, and agree to remove the private FreeType and Windows header. However, the cache is an internal LVGL thing. If the user wants all the LVGL stuff they can just include lvgl_private.h.

[FASTSHIFT]

I see the problem, and agree to remove the private FreeType and Windows header. However, the cache is an internal LVGL thing. If the user wants all the LVGL stuff they can just include lvgl_private.h.

OK, let's keep the cache private API.