privately link fmt, but propagate its header

[yxlao]

This issue was introduced in #4692.

After installation

➜  ~ tree open3d_install/include/open3d/3rdparty/fmt  
open3d_install/include/open3d/3rdparty/fmt
├── chrono.h
├── color.h
├── compile.h
├── core.h
├── format.h
├── format-inl.h
├── locale.h
├── ostream.h
├── posix.h
├── printf.h
├── ranges.h
└── safe-duration-cast.h

0 directories, 12 files
➜  ~ tree open3d_install/lib                        
open3d_install/lib
├── cmake
│   └── Open3D
│       ├── Open3DConfig.cmake
│       ├── Open3DConfigVersion.cmake
│       ├── Open3DTargets.cmake
│       └── Open3DTargets-relwithdebinfo.cmake
└── libOpen3D.so

2 directories, 5 files

This change is 

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.