libigl: dont install opengl headers

fix meshlab on darwin:
```
[ 47%] Building CXX object src/meshlabplugins/filter_cubization/CMakeFiles/filter_cubization.dir/filter_cubization_autogen/mocs_compilation.cpp.o
In file included from /tmp/nix-build-meshlab-2025.07.drv-0/source/build/src/meshlabplugins/filter_cubization/filter_cubization_autogen/mocs_compilation.cpp:2:
In file included from /tmp/nix-build-meshlab-2025.07.drv-0/source/build/src/meshlabplugins/filter_cubization/filter_cubization_autogen/EWIEGA46WW/moc_filter_cubization.cpp:10:
In file included from /tmp/nix-build-meshlab-2025.07.drv-0/source/build/src/meshlabplugins/filter_cubization/filter_cubization_autogen/EWIEGA46WW/../../../../../../src/meshlabplugins/filter_cubization/filter_cubization.h:28:
In file included from /tmp/nix-build-meshlab-2025.07.drv-0/source/src/common/../common/plugins/interfaces/filter_plugin.h:29:
In file included from /tmp/nix-build-meshlab-2025.07.drv-0/source/src/common/../common/plugins/interfaces/../../ml_document/mesh_document.h:27:
In file included from /tmp/nix-build-meshlab-2025.07.drv-0/source/src/common/../common/plugins/interfaces/../../ml_document/mesh_model.h:26:
In file included from /nix/store/18nlfi993h20y6dd1kfg8zm9gaglwygn-glew-2.2.0-dev/include/GL/glew.h:1217:
In file included from /nix/store/rhlyld20zdzdi5wghb4b12gw52s9whr9-apple-sdk-11.3/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/OpenGL.framework/Headers/glu.h:9:
/nix/store/zchkzj4md6q4lq09iidcm4c8n7r22k46-libigl-2.6.0/include/igl/OpenGL/gl.h:23:10: fatal error: 'glad/glad.h' file not found
   23 | #include <glad/glad.h>
      |          ^~~~~~~~~~~~~
```

pkgs/by-name/li/libigl/package.nix

@@ -20,6 +20,7 @@ stdenvNoCC.mkDerivation (finalAttrs: {
   installPhase = ''
     mkdir -p $out/include
     cp -r include/igl $out/include
+    rm -rf $out/include/igl/opengl
   '';
 
   meta = with lib; {