mapi: Hide OpenGL functions to be exported when shared-glapi is disabled

mapi: Hide OpenGL functions to be exported when shared-glapi is disabled

Fixes the following test error:
135/154 mesa:gallium / osmesa-symbols-check                                                                       FAIL             0.07s   exit status 1

```
src/gallium/targets/osmesa/libOSMesa.so.8.0.0: unknown symbol exported: glAreTexturesResidentEXT
src/gallium/targets/osmesa/libOSMesa.so.8.0.0: unknown symbol exported: glDeleteTexturesEXT
src/gallium/targets/osmesa/libOSMesa.so.8.0.0: unknown symbol exported: glGenTexturesEXT
src/gallium/targets/osmesa/libOSMesa.so.8.0.0: unknown symbol exported: glIsTextureEXT
```
The build options is:
```
-D glx=xlib -D gles1=disabled -D gles2=disabled -D shared-glapi=disabled
```

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Corentin Noël <corentin.noel@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23409>