Add "clib\__init__.py" in “python\opencc” (#693)

System description: Windows x64. Python version: 3.9.

Running "python setup.py build" results in the following error: 
```
copying python\opencc\__init__.py -> build\lib.win-amd64-3.9\opencc
package directory 'python\opencc\clib' does not exist.
```

This is because the package 'opencc.clib' is imported in 'python\opencc\__init__.py' (Line 6)  before the function "build_libopencc()" in 'setup.py' creates the directory "python\opencc\clib" (Line 83).

By adding the file "clib\__init__.py" in the directory “python\opencc”, the command "python setup.py build" can successfully build the files.

I do not know whether this is also related to the issue that the wheel files cannot be created on PyPI for Windows for the recent versions (>1.1.1)

python/opencc/clib/__init__.py

@@ -0,0 +1 @@
+