Add debug postfix for windows debug build

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**Describe the bug**
Compilation from source under Windows produces lib files with the same name for both debug and release builds.

**To Reproduce**
Compile from source with default configuration.

**Expected behavior**
It would be nice to fix the cmake to add the classical "d" debug postfix to debug libraries.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Environment (please complete the following information):**

- Operating system: Windows 10 64-bit)
- Python version: 3.8
- Open3D version: 0.13.0
- Is this remote workstation?: no
- How did you install Open3D?: build from source
- Compiler version (if built from source): Visual Studio 2019

**Additional context**
Add any other context about the problem here.