win32: The opengl headers should install to mesa/GL subdir

The subdir `mesa/GL` used to avoid conflict between Windows GL headers and mesa GL headers.
The GL headers resident in Windows SDK are:
```
gl.h
glaux.h
glcorearb.h
glext.h
glu.h
glxext.h
wgl.h
wglext.h
```

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
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merge request