Fix OpenGL symbol type mismatch warning at link time

[kcgen]

Description

Fixes the following symbol type mismatch issues flagged by the mold linker:

mold: warning: symbol type mismatch: glGenBuffersARB
>>> defined in src/gui/libgui.a(src/gui/libgui.a.p/sdlmain.cpp.o) as STT_OBJECT
>>> defined in /usr/lib/x86_64-linux-gnu/libGL.so as STT_FUNC
mold: warning: symbol type mismatch: glBufferDataARB
>>> defined in src/gui/libgui.a(src/gui/libgui.a.p/sdlmain.cpp.o) as STT_OBJECT
>>> defined in /usr/lib/x86_64-linux-gnu/libGL.so as STT_FUNC
mold: warning: symbol type mismatch: glDeleteBuffersARB
>>> defined in src/gui/libgui.a(src/gui/libgui.a.p/sdlmain.cpp.o) as STT_OBJECT
>>> defined in /usr/lib/x86_64-linux-gnu/libGL.so as STT_FUNC
mold: warning: symbol type mismatch: glUnmapBufferARB
>>> defined in src/gui/libgui.a(src/gui/libgui.a.p/sdlmain.cpp.o) as STT_OBJECT
>>> defined in /usr/lib/x86_64-linux-gnu/libGL.so as STT_FUNC
mold: warning: symbol type mismatch: glMapBufferARB
>>> defined in src/gui/libgui.a(src/gui/libgui.a.p/sdlmain.cpp.o) as STT_OBJECT
>>> defined in /usr/lib/x86_64-linux-gnu/libGL.so as STT_FUNC
mold: warning: symbol type mismatch: glBindBufferARB
>>> defined in src/gui/libgui.a(src/gui/libgui.a.p/sdlmain.cpp.o) as STT_OBJECT
>>> defined in /usr/lib/x86_64-linux-gnu/libGL.so as STT_FUNC

Related issues

None.

Manual testing

Tested OpenGL to confirm it's still working.

Checklist

I have:

• followed the project's contributing guidelines and code of conduct.
• performed a self-review of my code.
• commented on the particularly hard-to-understand areas of my code.
• split my work into well-defined, bisectable commits, and I named my commits well.
• applied the appropriate labels (bug, enhancement, refactoring, documentation, etc.)
• checked that all my commits can be built.
• confirmed that my code does not cause performance regressions (e.g., by running the Quake benchmark).
• added unit tests where applicable to prove the correctness of my code and to avoid future regressions.
• made corresponding changes to the documentation or the website according to the documentation guidelines.
• locally verified my website or documentation changes.

[kcgen]

Uses the same namespace approach we're using for the OpenGL 2.x API calls.

No new code; plan to merge when passing CI.