[Cmake]Fix undefined symbol for std::filesystem under GCC8 + Linux

[Aurelius84]

一、问题

在Linux机器GCC8版本以上，源码编译时正常，但import paddle2onnx 时会报如下错误：

  File "/workspace/pd2onnx-fork/paddle2onnx/convert.py", line 17, in <module>
    import paddle2onnx.paddle2onnx_cpp2py_export as c_p2o
ImportError: paddle2onnx_cpp2py_export.cpython-39-x86_64-linux-gnu.so: 
                      undefined symbol: _ZNKSt10filesystem7__cxx114path11parent_pathEv

二、修复

参考：https://discourse.cmake.org/t/correct-way-to-link-std-filesystem-with-gcc-8/4121/5

[Zheng-Bicheng]

LGTM